485BPOS

As filed with the U.S. Securities and Exchange Commission on January 9, 2013

Securities Act File No. 33-49552

Investment Company Act File No. 811-6740

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933	x
Pre-Effective Amendment No.	¨
Post-Effective Amendment No. 82	x

and/or

REGISTRATION STATEMENT

UNDER

THE INVESTMENT COMPANY ACT OF 1940 x

Amendment No. 88

(Check appropriate box or boxes)

Legg Mason Partners Institutional Trust*

(Exact Name of Registrant as Specified in Charter)

55 Water Street, New York, New York 10041

(Address of Principal Executive Offices) (Zip Code)

Registrant’s Telephone Number, including Area Code (877) 721-1926

Robert I. Frenkel

Legg Mason Partners Institutional Trust

100 First Stamford Place

Stamford, Connecticut 06902

(Name and Address of Agent for Service)

COPY TO:

Roger P. Joseph, Esq.

Bingham McCutchen LLP

One Federal Street

Boston, Massachusetts 02110

Continuous

(Approximate Date of Proposed Offering)

It is proposed that this filing will become effective:

x immediately upon filing pursuant to paragraph (b)

¨ on            pursuant to paragraph (b)

¨ 60 days after filing pursuant to paragraph (a)(1)

¨ on            pursuant to paragraph (a)(1)

¨ 75 days after filing pursuant to paragraph (a)(2)

¨ on            pursuant to paragraph (a)(2) of Rule 485.

If appropriate, check the following box:

¨ This post-effective amendment designates a new effective date for a previously filed post-effective amendment.

* This filing relates solely to Western Asset Institutional Liquid Reserves, Western Asset Institutional Cash Reserves, Western Asset Institutional U.S. Treasury Reserves, Western Asset Institutional Government Reserves, Western Asset Institutional Tax Free Reserves and Western Asset Institutional AMT Free Municipal Money Market Fund.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended (the “Securities Act”), and the Investment Company Act of 1940, as amended, the Registrant, LEGG MASON PARTNERS INSTITUTIONAL TRUST, certifies that it meets all requirements for effectiveness of this Post-Effective Amendment to the Registration Statement pursuant to Rule 485(b) under the Securities Act and has duly caused this Post-Effective Amendment to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of New York, State of New York on this 9th day of January, 2013.

LEGG MASON PARTNERS INSTITUTIONAL TRUST, on behalf of its series:

Western Asset Institutional Liquid Reserves

Western Asset Institutional Cash Reserves

Western Asset Institutional U.S. Treasury Reserves

Western Asset Institutional Government Reserves

Western Asset Institutional Tax Free Reserves

Western Asset Institutional AMT Free Municipal Money Market Fund

By:	/s/ R. Jay Gerken
	R. Jay Gerken
	President and Principal Executive Officer

WITNESS our hands on the date set forth below.

Pursuant to the requirements of the Securities Act, this Post-Effective Amendment to the Registration Statement has been signed below by the following persons in the capacities indicated below on January 9, 2013.

Signature	Title
/s/ R. Jay Gerken	President, Principal Executive Officer and Trustee
R. Jay Gerken
/s/ Richard F. Sennett	Principal Financial Officer
Richard F. Sennett
/s/ Elliott J. Berv*	Trustee
Elliott J. Berv
/s/ A. Benton Cocanougher*	Trustee
A. Benton Cocanougher
/s/ Jane F. Dasher*	Trustee
Jane F. Dasher
/s/ Mark T. Finn*	Trustee
Mark T. Finn
/s/ Stephen R. Gross*	Trustee
Stephen R. Gross

/s/ Richard E. Hanson, Jr.*	Trustee
Richard E. Hanson, Jr.
/s/ Diana R. Harrington*	Trustee
Diana R. Harrington
/s/ Susan M. Heilbron*	Trustee
Susan M. Heilbron
/s/ Susan B. Kerley*	Trustee
Susan B. Kerley
/s/ Alan G. Merten*	Trustee
Alan G. Merten
/s/ R. Richardson Pettit*	Trustee
R. Richardson Pettit

*By:	/s/ R. Jay Gerken
	R. Jay Gerken

* Attorney-in-Fact, pursuant to Power of Attorney.

Master Portfolio Trust has duly caused this Post-Effective Amendment to the Registration Statement on Form N-1A of Legg Mason Partners Institutional Trust to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of New York, State of New York on this 9th day of January, 2013.

MASTER PORTFOLIO TRUST, on behalf of its series, Liquid Reserves Portfolio, Prime Cash Reserves Portfolio, U.S. Treasury Reserves Portfolio, Government Portfolio and Tax Free Reserves Portfolio.

By:	/s/ R. Jay Gerken
	R. Jay Gerken
	President and Principal Executive Officer

WITNESS our hands on the date set forth below.

This Post-Effective Amendment to the Registration Statement has been signed below by the following persons in the capacities indicated below on January 9, 2013.

Signature	Title
/s/ R. Jay Gerken R. Jay Gerken	President, Principal Executive Officer and Trustee
/s/ Richard F. Sennett Richard F. Sennett	Principal Financial Officer
/s/ Elliott J. Berv* Elliott J. Berv	Trustee
/s/ A. Benton Cocanougher* A. Benton Cocanougher	Trustee
/s/ Jane F. Dasher* Jane F. Dasher	Trustee
/s/ Mark T. Finn* Mark T. Finn	Trustee
/s/ Stephen R. Gross* Stephen R. Gross	Trustee
/s/ Richard E. Hanson, Jr.* Richard E. Hanson, Jr.	Trustee
/s/ Diana R. Harrington* Diana R. Harrington	Trustee
/s/ Susan M. Heilbron* Susan M. Heilbron	Trustee
/s/ Susan B. Kerley* Susan B. Kerley	Trustee
/s/ Alan G. Merten* Alan G. Merten	Trustee

/s/ R. Richardson Pettit* R. Richardson Pettit	Trustee
*By: /s/ R. Jay Gerken R. Jay Gerken

* Attorney-in-Fact, pursuant to Power of Attorney.

EXHIBIT INDEX

Index No.	Description of Exhibit
EX-101.INS	XBRL Instance Document
EX-101.SCH	XBRL Taxonomy Extension Schema Document
EX-101.CAL	XBRL Taxonomy Extension Calculation Linkbase
EX-101.DEF	XBRL Taxonomy Extension Definition Linkbase
EX-101.LAB	XBRL Taxonomy Extension Labels Linkbase
EX-101.PRE	XBRL Taxonomy Extension Presentation Linkbase

XML

S Shares | Western Asset Institutional Cash Reserves

Western Asset Institutional Cash Reserves

Investment objective

The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Class S shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	S Shares Western Asset Institutional Cash Reserves Class S
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		S Shares Western Asset Institutional Cash Reserves Class S
Management fees		0.20%
Distribution and service (12b-1) fees		0.25%
Other expenses		0.03%
Total annual fund operating expenses		0.48%
Fees waived and/or expenses reimbursed	[1]	(0.03%)
Total annual fund operating expenses after waiving fees and/or reimbursing expenses		0.45%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
S Shares Western Asset Institutional Cash Reserves Class S	46	148	263	598

The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Class S shares. The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.27

Worst quarter
(03/31/2010): 0.00

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.01

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
S Shares Western Asset Institutional Cash Reserves Class S	0.02%	1.63%	1.94%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.45% for Class S shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Institutional Liquid Reserves Shares | Western Asset Institutional Liquid Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Liquid Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold SVB Securities Institutional Liquid Reserves Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2014
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Liquid Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Liquid Reserves Portfolio.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Liquid Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Liquid Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for SVB Securities Institutional Liquid Reserves Shares. The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available by calling your Service Agent at 1-800-303-7371. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for SVB Securities Institutional Liquid Reserves Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-800-303-7371
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.31 Worst quarter (09/30/2011): 0.01 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.09
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Institutional Liquid Reserves Shares | Western Asset Institutional Liquid Reserves | SVB Securities Institutional Liquid Reserves Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.21%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.10%
Other expenses	rr_OtherExpensesOverAssets	0.02%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.33%
Fees waived and/or expenses reimbursed	rr_FeeWaiverOrReimbursementOverAssets	(0.09%)	[1]
Total annual fund operating expenses after waiving fees and/or reimbursing expenses	rr_NetExpensesOverAssets	0.24%
1 year	rr_ExpenseExampleYear01	25
3 years	rr_ExpenseExampleYear03	88
5 years	rr_ExpenseExampleYear05	168
10 years	rr_ExpenseExampleYear10	402
2002	rr_AnnualReturn2002	1.81%
2003	rr_AnnualReturn2003	1.06%
2004	rr_AnnualReturn2004	1.26%
2005	rr_AnnualReturn2005	3.12%
2006	rr_AnnualReturn2006	4.89%
2007	rr_AnnualReturn2007	5.21%
2008	rr_AnnualReturn2008	3.03%
2009	rr_AnnualReturn2009	0.58%
2010	rr_AnnualReturn2010	0.13%
2011	rr_AnnualReturn2011	0.09%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.09%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.31%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.01%
1 year	rr_AverageAnnualReturnYear01	0.09%
5 years	rr_AverageAnnualReturnYear05	1.79%
10 years	rr_AverageAnnualReturnYear10	2.10%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.24% for SVB Securities Institutional Liquid Reserves Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Administrative | Western Asset Institutional AMT Free Municipal Money Market Fund

Western Asset Institutional AMT Free Municipal Money Market Fund

Investment objective

The fund seeks current income that is exempt from federal income tax to the extent consistent with preservation of capital and the maintenance of liquidity.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Administrative Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Administrative Western Asset Institutional AMT Free Municipal Money Market Fund Administrative Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Administrative Western Asset Institutional AMT Free Municipal Money Market Fund Administrative Shares
Management fees		0.25%
Distribution and service (12b-1) fees		0.20%
Other expenses	[1]	0.06%
Total annual fund operating expenses		0.51%
Fees waived and/or expenses reimbursed	[2]	(0.11%)
Total annual fund operating expenses after waiving fees and/or reimbursing expenses		0.40%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Administrative Western Asset Institutional AMT Free Municipal Money Market Fund Administrative Shares	41	141	263	619

Principal investment strategies

The fund is a money market fund which, under normal circumstances, will invest substantially all of its assets in short-term high quality municipal securities whose interest is exempt from federal income tax, including the federal alternative minimum tax. Municipal securities include debt obligations issued by any of the 50 states and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities and other structured securities such as variable rate demand obligations, tender option bonds, partnership interests and swap-based securities, many of which may be issued or backed by U.S. or non-U.S. banks.

Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category (or, with respect to not more than 3% of its total assets, in the second highest category) or, if not rated, are determined by the subadviser to be of equivalent quality.

The interest paid on municipal securities purchased by the fund is excluded from gross income for regular federal income tax purposes, and, as noted above, will not typically be subject to the federal alternative minimum tax. As a result, the rate of interest paid on these securities normally is lower than the rate of interest paid on fully taxable securities.

Under normal circumstances, the fund must invest at least 80% of its net assets in short-term high quality municipal obligations and interests in municipal obligations that pay interest that is exempt from federal income tax, including the federal alternative minimum tax. Although the fund will ordinarily not invest in securities the interest on which is subject to regular federal income tax or the federal alternative minimum tax, when the subadviser believes that suitable AMT-free municipal securities are not available, the fund may temporarily invest up to 20% of its assets in investments that pay interest that may be subject to federal income tax or the federal alternative minimum tax. In addition, the fund may depart from its principal investment strategies in response to unusually adverse market, economic or political conditions, as described under “More on the funds’ investment strategies, investments and risks – Defensive investing.”

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term municipal debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated.

Tax risk. The income on the fund’s municipal securities could become subject to federal income tax due to noncompliant conduct by issuers, unfavorable legislation or litigation or adverse interpretations by regulatory authorities.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Risks relating to investments in municipal securities. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(09/30/2007): 0.88

Worst quarter
(03/31/2010): 0.02

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.04

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Administrative Western Asset Institutional AMT Free Municipal Money Market Fund Institutional Shares	0.16%	1.27%	1.51%

[1] "Other expenses" for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

[2] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.40% for Administrative Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Investor | Western Asset Institutional Tax Free Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Tax Free Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objectives are to provide shareholders with high levels of current income exempt from federal income taxes, preservation of capital and liquidity.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Investor Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2014
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Tax Free Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Tax Free Reserves Portfolio.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other expenses” for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Tax Free Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Tax Free Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which, under normal market conditions, invests at least 80% of its assets in short-term high quality municipal obligations and interests in municipal obligations that pay interest that is exempt from federal income tax, including the federal alternative minimum tax. Municipal securities include debt obligations issued by any of the 50 states and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities and other structured securities such as variable rate demand obligations, tender option bonds, partnership interests and swap-based securities, many of which may be issued or backed by U.S. or non-U.S. banks. Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category (or, with respect to not more than 3% of its total assets, in the second highest category) or, if not rated, are determined by the subadviser to be of equivalent quality. Under normal circumstances, the fund may invest up to 20% of its assets in investments that pay interest that may be subject to regular federal income tax and/or the federal alternative minimum tax, although for temporary or defensive purposes, the fund may invest an unlimited amount in such securities. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term municipal debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated. Tax risk. The income on the fund’s municipal securities could become subject to federal income tax due to noncompliant conduct by issuers, unfavorable legislation or litigation or adverse interpretations by regulatory authorities. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Risks relating to investments in municipal securities. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”). The fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund. However, the fund intends to comply with the diversification requirements applicable to money market funds which limit the fund’s ability to invest in the obligations of a single issuer.
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Nondiversified Status [Text]	rr_RiskNondiversifiedStatus	The fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund. However, the fund intends to comply with the diversification requirements applicable to money market funds which limit the fund’s ability to invest in the obligations of a single issuer.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Investor Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (06/30/2007): 0.90 Worst quarter (12/31/2011): 0.02 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.05
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ.
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Investor | Western Asset Institutional Tax Free Reserves | Investor Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.24%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.10%
Other expenses	rr_OtherExpensesOverAssets	0.05%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.39%
Fees waived and/or expenses reimbursed	rr_FeeWaiverOrReimbursementOverAssets	(0.04%)	[2]
Total annual fund operating expenses after waiving fees and/or reimbursing expenses	rr_NetExpensesOverAssets	0.35%
1 year	rr_ExpenseExampleYear01	36
3 years	rr_ExpenseExampleYear03	117
5 years	rr_ExpenseExampleYear05	211
10 years	rr_ExpenseExampleYear10	485
Investor | Western Asset Institutional Tax Free Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
2002	rr_AnnualReturn2002	1.40%
2003	rr_AnnualReturn2003	0.89%
2004	rr_AnnualReturn2004	1.02%
2005	rr_AnnualReturn2005	2.25%
2006	rr_AnnualReturn2006	3.27%
2007	rr_AnnualReturn2007	3.50%
2008	rr_AnnualReturn2008	2.13%
2009	rr_AnnualReturn2009	0.41%
2010	rr_AnnualReturn2010	0.18%
2011	rr_AnnualReturn2011	0.16%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.05%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2007
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.90%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.02%
1 Year	rr_AverageAnnualReturnYear01	0.16%
5 Years	rr_AverageAnnualReturnYear05	1.27%
10 Years	rr_AverageAnnualReturnYear10	1.51%

[1] "Other expenses" for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

[2] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.35% for Investor Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees’ consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class’ total annual operating expenses have fallen to a level below the limit described above.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Premium | Western Asset Institutional Government Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Government Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund seeks maximum current income to the extent consistent with preservation of capital and the maintenance of liquidity.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Premium Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Government Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Government Portfolio.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other expenses” for Premium Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Government Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Government Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests exclusively in short-term U.S. government obligations, including U.S. Treasuries and securities issued or guaranteed by the U.S. government or its agencies, authorities, instrumentalities or sponsored entities and in repurchase agreements. These securities may pay interest at fixed, floating or adjustable rates or may be issued at a discount. U.S. government obligations are not necessarily backed by the full faith and credit of the United States. Although the fund invests in U.S. government obligations, an investment in the fund is neither insured nor guaranteed by the U.S. government. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Premium Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Premium Shares because there were no Premium Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Premium Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.28 Worst quarter (09/30/2011): 0.01 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.03
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Premium Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Premium Shares because there were no Premium Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ.
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Premium | Western Asset Institutional Government Reserves | Premium Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.20%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.10%
Other expenses	rr_OtherExpensesOverAssets	0.02%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.32%
1 year	rr_ExpenseExampleYear01	33
3 years	rr_ExpenseExampleYear03	103
5 years	rr_ExpenseExampleYear05	180
10 years	rr_ExpenseExampleYear10	405
Premium | Western Asset Institutional Government Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
2002	rr_AnnualReturn2002	1.54%
2003	rr_AnnualReturn2003	0.95%
2004	rr_AnnualReturn2004	1.16%
2005	rr_AnnualReturn2005	3.04%
2006	rr_AnnualReturn2006	4.83%
2007	rr_AnnualReturn2007	5.02%
2008	rr_AnnualReturn2008	2.46%
2009	rr_AnnualReturn2009	0.39%
2010	rr_AnnualReturn2010	0.09%
2011	rr_AnnualReturn2011	0.04%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.03%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.28%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.01%
1 year	rr_AverageAnnualReturnYear01	0.04%
5 years	rr_AverageAnnualReturnYear05	1.58%
10 years	rr_AverageAnnualReturnYear10	1.94%

[1] "Other expenses" for Premium Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

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Horizon Shares | Western Asset Institutional Cash Reserves

Western Asset Institutional Cash Reserves

Investment objective

The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold SVB Securities Horizon Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Horizon Shares Western Asset Institutional Cash Reserves SVB Securities Horizon Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Horizon Shares Western Asset Institutional Cash Reserves SVB Securities Horizon Shares
Management fees		0.20%
Distribution and service (12b-1) fees		0.27%
Other expenses		0.03%
Total annual fund operating expenses		0.50%
Fees waived and/or expenses reimbursed	[1]	(0.05%)
Total annual fund operating expenses after waiving fees and/or reimbursing expenses		0.45%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Horizon Shares Western Asset Institutional Cash Reserves SVB Securities Horizon Shares	46	150	269	619

The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for SVB Securities Horizon Shares. The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available by calling your Service Agent at 1-800-303-7371.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.27

Worst quarter
(03/31/2010): 0.01

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.01

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Horizon Shares Western Asset Institutional Cash Reserves SVB Securities Horizon Shares	0.02%	1.64%	1.87%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.45% for SVB Securities Horizon Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

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Investor | Western Asset Institutional AMT Free Municipal Money Market Fund

Western Asset Institutional AMT Free Municipal Money Market Fund

Investment objective

The fund seeks current income that is exempt from federal income tax to the extent consistent with preservation of capital and the maintenance of liquidity.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Investor Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Investor Western Asset Institutional AMT Free Municipal Money Market Fund Investor Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Investor Western Asset Institutional AMT Free Municipal Money Market Fund Investor Shares
Management fees		0.25%
Distribution and service (12b-1) fees		0.10%
Other expenses	[1]	0.06%
Total annual fund operating expenses		0.41%
Fees waived and/or expenses reimbursed	[2]	(0.06%)
Total annual fund operating expenses after waiving fees and/or reimbursing expenses		0.35%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Investor Western Asset Institutional AMT Free Municipal Money Market Fund Investor Shares	36	119	217	505

Principal investment strategies

The fund is a money market fund which, under normal circumstances, will invest substantially all of its assets in short-term high quality municipal securities whose interest is exempt from federal income tax, including the federal alternative minimum tax. Municipal securities include debt obligations issued by any of the 50 states and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities and other structured securities such as variable rate demand obligations, tender option bonds, partnership interests and swap-based securities, many of which may be issued or backed by U.S. or non-U.S. banks.

Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category (or, with respect to not more than 3% of its total assets, in the second highest category) or, if not rated, are determined by the subadviser to be of equivalent quality.

The interest paid on municipal securities purchased by the fund is excluded from gross income for regular federal income tax purposes, and, as noted above, will not typically be subject to the federal alternative minimum tax. As a result, the rate of interest paid on these securities normally is lower than the rate of interest paid on fully taxable securities.

Under normal circumstances, the fund must invest at least 80% of its net assets in short-term high quality municipal obligations and interests in municipal obligations that pay interest that is exempt from federal income tax, including the federal alternative minimum tax. Although the fund will ordinarily not invest in securities the interest on which is subject to regular federal income tax or the federal alternative minimum tax, when the subadviser believes that suitable AMT-free municipal securities are not available, the fund may temporarily invest up to 20% of its assets in investments that pay interest that may be subject to federal income tax or the federal alternative minimum tax. In addition, the fund may depart from its principal investment strategies in response to unusually adverse market, economic or political conditions, as described under “More on the funds’ investment strategies, investments and risks—Defensive investing.”

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term municipal debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated.

Tax risk. The income on the fund’s municipal securities could become subject to federal income tax due to noncompliant conduct by issuers, unfavorable legislation or litigation or adverse interpretations by regulatory authorities.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Risks relating to investments in municipal securities. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(09/30/2007): 0.88

Worst quarter
(03/31/2010): 0.02

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.04

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Investor Western Asset Institutional AMT Free Municipal Money Market Fund Institutional Shares	0.16%	1.27%	1.51%

[1] "Other expenses" for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

[2] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.35% for Investor Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees’ consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class’ total annual operating expenses have fallen to a level below the limit described above.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Administrative | Western Asset Institutional AMT Free Municipal Money Market Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional AMT Free Municipal Money Market Fund
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund seeks current income that is exempt from federal income tax to the extent consistent with preservation of capital and the maintenance of liquidity.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Administrative Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2014
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other expenses” for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which, under normal circumstances, will invest substantially all of its assets in short-term high quality municipal securities whose interest is exempt from federal income tax, including the federal alternative minimum tax. Municipal securities include debt obligations issued by any of the 50 states and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities and other structured securities such as variable rate demand obligations, tender option bonds, partnership interests and swap-based securities, many of which may be issued or backed by U.S. or non-U.S. banks. Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category (or, with respect to not more than 3% of its total assets, in the second highest category) or, if not rated, are determined by the subadviser to be of equivalent quality. The interest paid on municipal securities purchased by the fund is excluded from gross income for regular federal income tax purposes, and, as noted above, will not typically be subject to the federal alternative minimum tax. As a result, the rate of interest paid on these securities normally is lower than the rate of interest paid on fully taxable securities. Under normal circumstances, the fund must invest at least 80% of its net assets in short-term high quality municipal obligations and interests in municipal obligations that pay interest that is exempt from federal income tax, including the federal alternative minimum tax. Although the fund will ordinarily not invest in securities the interest on which is subject to regular federal income tax or the federal alternative minimum tax, when the subadviser believes that suitable AMT-free municipal securities are not available, the fund may temporarily invest up to 20% of its assets in investments that pay interest that may be subject to federal income tax or the federal alternative minimum tax. In addition, the fund may depart from its principal investment strategies in response to unusually adverse market, economic or political conditions, as described under “More on the funds’ investment strategies, investments and risks – Defensive investing.” As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term municipal debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated. Tax risk. The income on the fund’s municipal securities could become subject to federal income tax due to noncompliant conduct by issuers, unfavorable legislation or litigation or adverse interpretations by regulatory authorities. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Risks relating to investments in municipal securities. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (09/30/2007): 0.88 Worst quarter (03/31/2010): 0.02 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.04
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ.
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Administrative | Western Asset Institutional AMT Free Municipal Money Market Fund | Administrative Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.25%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.20%
Other expenses	rr_OtherExpensesOverAssets	0.06%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.51%
Fees waived and/or expenses reimbursed	rr_FeeWaiverOrReimbursementOverAssets	(0.11%)	[2]
Total annual fund operating expenses after waiving fees and/or reimbursing expenses	rr_NetExpensesOverAssets	0.40%
1 year	rr_ExpenseExampleYear01	41
3 years	rr_ExpenseExampleYear03	141
5 years	rr_ExpenseExampleYear05	263
10 years	rr_ExpenseExampleYear10	619
Administrative | Western Asset Institutional AMT Free Municipal Money Market Fund | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
2002	rr_AnnualReturn2002	1.34%
2003	rr_AnnualReturn2003	0.89%
2004	rr_AnnualReturn2004	1.04%
2005	rr_AnnualReturn2005	2.23%
2006	rr_AnnualReturn2006	3.27%
2007	rr_AnnualReturn2007	3.47%
2008	rr_AnnualReturn2008	2.16%
2009	rr_AnnualReturn2009	0.38%
2010	rr_AnnualReturn2010	0.19%
2011	rr_AnnualReturn2011	0.16%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.04%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2007
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.88%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2010
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.02%
1 year	rr_AverageAnnualReturnYear01	0.16%
5 years	rr_AverageAnnualReturnYear05	1.27%
10 years	rr_AverageAnnualReturnYear10	1.51%

[1] "Other expenses" for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

[2] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.40% for Administrative Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Investor | Western Asset Institutional Cash Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Cash Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Investor Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other expenses” for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SA”).
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class O shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class O shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.32 Worst quarter (09/30/2011): 0.03 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.15
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class O shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ.
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Investor | Western Asset Institutional Cash Reserves | Investor Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.20%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.10%
Other expenses	rr_OtherExpensesOverAssets	0.03%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.33%
1 year	rr_ExpenseExampleYear01	34
3 years	rr_ExpenseExampleYear03	106
5 years	rr_ExpenseExampleYear05	186
10 years	rr_ExpenseExampleYear10	418
Investor | Western Asset Institutional Cash Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
2003	rr_AnnualReturn2003	1.08%
2004	rr_AnnualReturn2004	1.30%
2005	rr_AnnualReturn2005	3.19%
2006	rr_AnnualReturn2006	5.02%
2007	rr_AnnualReturn2007	5.27%
2008	rr_AnnualReturn2008	2.98%
2009	rr_AnnualReturn2009	0.62%
2010	rr_AnnualReturn2010	0.21%
2011	rr_AnnualReturn2011	0.17%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.15%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.32%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.03%
1 year	rr_AverageAnnualReturnYear01	0.17%
5 years	rr_AverageAnnualReturnYear05	1.83%
Since Inception	rr_AverageAnnualReturnSinceInception	2.17%
Inception date	rr_AverageAnnualReturnInceptionDate	Oct. 02, 2002

[1] "Other expenses" for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
L Shares | Western Asset Institutional Cash Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Cash Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Class L shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2014
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Class L shares. The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Class L shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006):1.31 Worst quarter (09/30/2011): 0.02 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.11
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
L Shares | Western Asset Institutional Cash Reserves | Class L
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.20%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.10%
Other expenses	rr_OtherExpensesOverAssets	0.03%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.33%
Fees waived and/or expenses reimbursed	rr_FeeWaiverOrReimbursementOverAssets	(0.03%)	[1]
Total annual fund operating expenses after waiving fees and/or reimbursing expenses	rr_NetExpensesOverAssets	0.30%
1 year	rr_ExpenseExampleYear01	31
3 years	rr_ExpenseExampleYear03	100
5 years	rr_ExpenseExampleYear05	180
10 years	rr_ExpenseExampleYear10	413
2002	rr_AnnualReturn2002	1.64%
2003	rr_AnnualReturn2003	1.03%
2004	rr_AnnualReturn2004	1.25%
2005	rr_AnnualReturn2005	3.14%
2006	rr_AnnualReturn2006	4.96%
2007	rr_AnnualReturn2007	5.22%
2008	rr_AnnualReturn2008	2.92%
2009	rr_AnnualReturn2009	0.58%
2010	rr_AnnualReturn2010	0.17%
2011	rr_AnnualReturn2011	0.12%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.11%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.31%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.02%
1 year	rr_AverageAnnualReturnYear01	0.12%
5 years	rr_AverageAnnualReturnYear05	1.78%
10 years	rr_AverageAnnualReturnYear10	2.09%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.30% for Class L shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
S Shares | Western Asset Institutional Cash Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Cash Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Class S shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2014
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Class S shares. The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Class S shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.27 Worst quarter (03/31/2010): 0.00 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.01
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
S Shares | Western Asset Institutional Cash Reserves | Class S
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.20%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other expenses	rr_OtherExpensesOverAssets	0.03%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.48%
Fees waived and/or expenses reimbursed	rr_FeeWaiverOrReimbursementOverAssets	(0.03%)	[1]
Total annual fund operating expenses after waiving fees and/or reimbursing expenses	rr_NetExpensesOverAssets	0.45%
1 year	rr_ExpenseExampleYear01	46
3 years	rr_ExpenseExampleYear03	148
5 years	rr_ExpenseExampleYear05	263
10 years	rr_ExpenseExampleYear10	598
2002	rr_AnnualReturn2002	1.49%
2003	rr_AnnualReturn2003	0.87%
2004	rr_AnnualReturn2004	1.09%
2005	rr_AnnualReturn2005	2.98%
2006	rr_AnnualReturn2006	4.81%
2007	rr_AnnualReturn2007	5.06%
2008	rr_AnnualReturn2008	2.77%
2009	rr_AnnualReturn2009	0.39%
2010	rr_AnnualReturn2010	0.03%
2011	rr_AnnualReturn2011	0.02%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.01%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.27%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2010
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.00%
1 year	rr_AverageAnnualReturnYear01	0.02%
5 years	rr_AverageAnnualReturnYear05	1.63%
10 years	rr_AverageAnnualReturnYear10	1.94%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.45% for Class S shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Administrative | Western Asset Institutional Liquid Reserves

Western Asset Institutional Liquid Reserves

Investment objective

The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Administrative Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Administrative Western Asset Institutional Liquid Reserves Administrative Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Administrative Western Asset Institutional Liquid Reserves Administrative Shares
Management fees		0.21%
Distribution and service (12b-1) fees		0.20%
Other expenses	[1]	0.02%
Total annual fund operating expenses		0.43%
Fees waived and/or expenses reimbursed	[2]	(0.03%)
Total annual fund operating expenses after waiving fees and/or reimbursing expenses		0.40%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund's operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Administrative Western Asset Institutional Liquid Reserves Administrative Shares	41	132	235	537

The fund invests in securities through an underlying fund: Liquid Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Liquid Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.34

Worst quarter
(09/30/2011): 0.03

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.15

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Administrative Western Asset Institutional Liquid Reserves Institutional Shares	0.17%	1.88%	2.18%

[1] "Other expenses" for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

[2] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.40% for Administrative Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Premium | Western Asset Institutional Government Reserves

Western Asset Institutional Government Reserves

Investment objective

The fund seeks maximum current income to the extent consistent with preservation of capital and the maintenance of liquidity.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Premium Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Premium Western Asset Institutional Government Reserves Premium Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Premium Western Asset Institutional Government Reserves Premium Shares
Management fees		0.20%
Distribution and service (12b-1) fees		0.10%
Other expenses	[1]	0.02%
Total annual fund operating expenses		0.32%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Premium Western Asset Institutional Government Reserves Premium Shares	33	103	180	405

The fund invests in securities through an underlying fund: Government Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Government Portfolio.

Principal investment strategies

The fund is a money market fund which invests exclusively in short-term U.S. government obligations, including U.S. Treasuries and securities issued or guaranteed by the U.S. government or its agencies, authorities, instrumentalities or sponsored entities and in repurchase agreements. These securities may pay interest at fixed, floating or adjustable rates or may be issued at a discount. U.S. government obligations are not necessarily backed by the full faith and credit of the United States. Although the fund invests in U.S. government obligations, an investment in the fund is neither insured nor guaranteed by the U.S. government.

The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Premium Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Premium Shares because there were no Premium Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.28

Worst quarter
(09/30/2011): 0.01

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.03

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Premium Western Asset Institutional Government Reserves Institutional Shares	0.04%	1.58%	1.94%

[1] "Other expenses" for Premium Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Investor | Western Asset Institutional Government Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Government Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund seeks maximum current income to the extent consistent with preservation of capital and the maintenance of liquidity.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Investor Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Government Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Government Portfolio.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other expenses” for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Government Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Government Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests exclusively in short-term U.S. government obligations, including U.S. Treasuries and securities issued or guaranteed by the U.S. government or its agencies, authorities, instrumentalities or sponsored entities and in repurchase agreements. These securities may pay interest at fixed, floating or adjustable rates or may be issued at a discount. U.S. government obligations are not necessarily backed by the full faith and credit of the United States. Although the fund invests in U.S. government obligations, an investment in the fund is neither insured nor guaranteed by the U.S. government. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.28 Worst quarter (09/30/2011): 0.01 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.03
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ.
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Investor | Western Asset Institutional Government Reserves | Investor Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.20%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.10%
Other expenses	rr_OtherExpensesOverAssets	0.02%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.32%
1 year	rr_ExpenseExampleYear01	33
3 years	rr_ExpenseExampleYear03	103
5 years	rr_ExpenseExampleYear05	180
10 years	rr_ExpenseExampleYear10	405
Investor | Western Asset Institutional Government Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
2002	rr_AnnualReturn2002	1.54%
2003	rr_AnnualReturn2003	0.95%
2004	rr_AnnualReturn2004	1.16%
2005	rr_AnnualReturn2005	3.04%
2006	rr_AnnualReturn2006	4.83%
2007	rr_AnnualReturn2007	5.02%
2008	rr_AnnualReturn2008	2.46%
2009	rr_AnnualReturn2009	0.39%
2010	rr_AnnualReturn2010	0.09%
2011	rr_AnnualReturn2011	0.04%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.03%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.28%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.01%
1 year	rr_AverageAnnualReturnYear01	0.04%
5 years	rr_AverageAnnualReturnYear05	1.58%
10 years	rr_AverageAnnualReturnYear10	1.94%

[1] "Other expenses" for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Premium | Western Asset Institutional Cash Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Cash Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Premium Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other expenses” for Premium Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Class L shares, a class of the fund that is invested in the same portfolio of securities as Premium Shares. The table shows the average annual total returns of Class L shares. No performance information is presented for Premium Shares because there were no Premium Shares outstanding during the periods shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Class L shares, a class of the fund that is invested in the same portfolio of securities as Premium Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.31 Worst quarter (09/30/2011): 0.02 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.11
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The bar chart shows changes in the fund’s performance from year to year for Class L shares, a class of the fund that is invested in the same portfolio of securities as Premium Shares. The table shows the average annual total returns of Class L shares. No performance information is presented for Premium Shares because there were no Premium Shares outstanding during the periods shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ.
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Premium | Western Asset Institutional Cash Reserves | Premium Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.20%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.10%
Other expenses	rr_OtherExpensesOverAssets	0.03%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.33%
1 year	rr_ExpenseExampleYear01	34
3 years	rr_ExpenseExampleYear03	106
5 years	rr_ExpenseExampleYear05	186
10 years	rr_ExpenseExampleYear10	418
Premium | Western Asset Institutional Cash Reserves | Class L
Risk/Return:	rr_RiskReturnAbstract
2002	rr_AnnualReturn2002	1.64%
2003	rr_AnnualReturn2003	1.03%
2004	rr_AnnualReturn2004	1.25%
2005	rr_AnnualReturn2005	3.14%
2006	rr_AnnualReturn2006	4.96%
2007	rr_AnnualReturn2007	5.22%
2008	rr_AnnualReturn2008	2.92%
2009	rr_AnnualReturn2009	0.58%
2010	rr_AnnualReturn2010	0.17%
2011	rr_AnnualReturn2011	0.12%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.11%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.31%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.02%
1 year	rr_AverageAnnualReturnYear01	0.12%
5 years	rr_AverageAnnualReturnYear05	1.78%
10 years	rr_AverageAnnualReturnYear10	2.09%

[1] "Other expenses" for Premium Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Investor | Western Asset Institutional Liquid Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Liquid Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Investor Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Liquid Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Liquid Reserves Portfolio.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other expenses” for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund's operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Liquid Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Liquid Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.34 Worst quarter (09/30/2011): 0.03 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.15
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ.
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Investor | Western Asset Institutional Liquid Reserves | Investor Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.21%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.10%
Other expenses	rr_OtherExpensesOverAssets	0.02%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.33%
1 year	rr_ExpenseExampleYear01	34
3 years	rr_ExpenseExampleYear03	106
5 years	rr_ExpenseExampleYear05	186
10 years	rr_ExpenseExampleYear10	418
Investor | Western Asset Institutional Liquid Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
2002	rr_AnnualReturn2002	1.85%
2003	rr_AnnualReturn2003	1.10%
2004	rr_AnnualReturn2004	1.31%
2005	rr_AnnualReturn2005	3.20%
2006	rr_AnnualReturn2006	4.99%
2007	rr_AnnualReturn2007	5.32%
2008	rr_AnnualReturn2008	3.11%
2009	rr_AnnualReturn2009	0.68%
2010	rr_AnnualReturn2010	0.23%
2011	rr_AnnualReturn2011	0.17%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.15%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.34%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.03%
1 Year	rr_AverageAnnualReturnYear01	0.17%
5 Years	rr_AverageAnnualReturnYear05	1.88%
10 Years	rr_AverageAnnualReturnYear10	2.18%

[1] "Other expenses" for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

XML

Premium | Western Asset Institutional AMT Free Municipal Money Market Fund

Western Asset Institutional AMT Free Municipal Money Market Fund

Investment objective

The fund seeks current income that is exempt from federal income tax to the extent consistent with preservation of capital and the maintenance of liquidity.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Premium Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Premium Western Asset Institutional AMT Free Municipal Money Market Fund Premium Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Premium Western Asset Institutional AMT Free Municipal Money Market Fund Premium Shares
Management fees		0.25%
Distribution and service (12b-1) fees		0.10%
Other expenses	[1]	0.06%
Total annual fund operating expenses		0.41%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Premium Western Asset Institutional AMT Free Municipal Money Market Fund Premium Shares	42	132	230	517

Principal investment strategies

The fund is a money market fund which, under normal circumstances, will invest substantially all of its assets in short-term high quality municipal securities whose interest is exempt from federal income tax, including the federal alternative minimum tax. Municipal securities include debt obligations issued by any of the 50 states and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities and other structured securities such as variable rate demand obligations, tender option bonds, partnership interests and swap-based securities, many of which may be issued or backed by U.S. or non-U.S. banks.

Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category (or, with respect to not more than 3% of its total assets, in the second highest category) or, if not rated, are determined by the subadviser to be of equivalent quality.

The interest paid on municipal securities purchased by the fund is excluded from gross income for regular federal income tax purposes, and, as noted above, will not typically be subject to the federal alternative minimum tax. As a result, the rate of interest paid on these securities normally is lower than the rate of interest paid on fully taxable securities.

Under normal circumstances, the fund must invest at least 80% of its net assets in short-term high quality municipal obligations and interests in municipal obligations that pay interest that is exempt from federal income tax, including the federal alternative minimum tax. Although the fund will ordinarily not invest in securities the interest on which is subject to regular federal income tax or the federal alternative minimum tax, when the subadviser believes that suitable AMT-free municipal securities are not available, the fund may temporarily invest up to 20% of its assets in investments that pay interest that may be subject to federal income tax or the federal alternative minimum tax. In addition, the fund may depart from its principal investment strategies in response to unusually adverse market, economic or political conditions, as described under “More on the funds’ investment strategies, investments and risks—Defensive investing.”

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term municipal debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated.

Tax risk. The income on the fund’s municipal securities could become subject to federal income tax due to noncompliant conduct by issuers, unfavorable legislation or litigation or adverse interpretations by regulatory authorities.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Risks relating to investments in municipal securities. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Premium Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Premium Shares because there were no Premium Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(09/30/2007): 0.88

Worst quarter
(03/31/2010): 0.02

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.04

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Premium Western Asset Institutional AMT Free Municipal Money Market Fund Institutional Shares	0.16%	1.27%	1.51%

[1] "Other expenses" for Premium Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

XML

Institutional | Western Asset Institutional AMT Free Municipal Money Market Fund

Western Asset Institutional AMT Free Municipal Money Market Fund

Investment objective

The fund seeks current income that is exempt from federal income tax to the extent consistent with preservation of capital and the maintenance of liquidity.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Institutional Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Institutional Western Asset Institutional AMT Free Municipal Money Market Fund Institutional Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Institutional Western Asset Institutional AMT Free Municipal Money Market Fund Institutional Shares
Management fees		0.25%
Distribution and service (12b-1) fees		none
Other expenses		0.06%
Total annual fund operating expenses		0.31%
Fees waived and/or expenses reimbursed	[1]	(0.08%)
Total annual fund operating expenses after waiving fees and/or reimbursing expenses		0.23%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Institutional Western Asset Institutional AMT Free Municipal Money Market Fund Institutional Shares	24	84	158	378

Principal investment strategies

The fund is a money market fund which, under normal circumstances, will invest substantially all of its assets in short-term high quality municipal securities whose interest is exempt from federal income tax, including the federal alternative minimum tax. Municipal securities include debt obligations issued by any of the 50 states and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities and other structured securities such as variable rate demand obligations, tender option bonds, partnership interests and swap-based securities, many of which may be issued or backed by U.S. or non-U.S. banks.

Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category (or, with respect to not more than 3% of its total assets, in the second highest category) or, if not rated, are determined by the subadviser to be of equivalent quality.

The interest paid on municipal securities purchased by the fund is excluded from gross income for regular federal income tax purposes, and, as noted above, will not typically be subject to the federal alternative minimum tax. As a result, the rate of interest paid on these securities normally is lower than the rate of interest paid on fully taxable securities.

Under normal circumstances, the fund must invest at least 80% of its net assets in short-term high quality municipal obligations and interests in municipal obligations that pay interest that is exempt from federal income tax, including the federal alternative minimum tax. Although the fund will ordinarily not invest in securities the interest on which is subject to regular federal income tax or the federal alternative minimum tax, when the subadviser believes that suitable AMT-free municipal securities are not available, the fund may temporarily invest up to 20% of its assets in investments that pay interest that may be subject to federal income tax or the federal alternative minimum tax. In addition, the fund may depart from its principal investment strategies in response to unusually adverse market, economic or political conditions, as described under “More on the funds’ investment strategies, investments and risks—Defensive investing.”

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term municipal debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated.

Tax risk. The income on the fund’s municipal securities could become subject to federal income tax due to noncompliant conduct by issuers, unfavorable legislation or litigation or adverse interpretations by regulatory authorities.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Risks relating to investments in municipal securities. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected
by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares). The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(09/30/2007): 0.88

Worst quarter
(03/31/2010): 0.02

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.04

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Institutional Western Asset Institutional AMT Free Municipal Money Market Fund Institutional Shares	0.16%	1.27%	1.51%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.23% for Institutional Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Institutional | Western Asset Institutional Cash Reserves

Western Asset Institutional Cash Reserves

Investment objective

The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Institutional Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Institutional Western Asset Institutional Cash Reserves Institutional Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses	Institutional Western Asset Institutional Cash Reserves Institutional Shares
Management fees	0.20%
Distribution and service (12b-1) fees	none
Other expenses	0.02%
Total annual fund operating expenses	0.22%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Institutional Western Asset Institutional Cash Reserves Institutional Shares	23	72	125	281

The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class O shares). The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.32

Worst quarter
(09/30/2011): 0.03

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.15

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	Since Inception	Inception date
Institutional Western Asset Institutional Cash Reserves Institutional Shares	0.17%	1.83%	2.17%	Oct. 02, 2002

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Horizon Shares | Western Asset Institutional Cash Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Cash Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold SVB Securities Horizon Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2014
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for SVB Securities Horizon Shares. The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available by calling your Service Agent at 1-800-303-7371. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for SVB Securities Horizon Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-800-303-7371
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.27 Worst quarter (03/31/2010): 0.01 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.01
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Horizon Shares | Western Asset Institutional Cash Reserves | SVB Securities Horizon Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.20%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.27%
Other expenses	rr_OtherExpensesOverAssets	0.03%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.50%
Fees waived and/or expenses reimbursed	rr_FeeWaiverOrReimbursementOverAssets	(0.05%)	[1]
Total annual fund operating expenses after waiving fees and/or reimbursing expenses	rr_NetExpensesOverAssets	0.45%
1 year	rr_ExpenseExampleYear01	46
3 years	rr_ExpenseExampleYear03	150
5 years	rr_ExpenseExampleYear05	269
10 years	rr_ExpenseExampleYear10	619
2002	rr_AnnualReturn2002	0.68%
2003	rr_AnnualReturn2003	0.83%
2004	rr_AnnualReturn2004	1.14%
2005	rr_AnnualReturn2005	3.03%
2006	rr_AnnualReturn2006	4.83%
2007	rr_AnnualReturn2007	5.06%
2008	rr_AnnualReturn2008	2.77%
2009	rr_AnnualReturn2009	0.41%
2010	rr_AnnualReturn2010	0.05%
2011	rr_AnnualReturn2011	0.02%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.01%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.27%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2010
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.01%
1 year	rr_AverageAnnualReturnYear01	0.02%
5 years	rr_AverageAnnualReturnYear05	1.64%
10 years	rr_AverageAnnualReturnYear10	1.87%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.45% for SVB Securities Horizon Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

L Shares | Western Asset Institutional Cash Reserves

Western Asset Institutional Cash Reserves

Investment objective

The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Class L shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	L Shares Western Asset Institutional Cash Reserves Class L
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		L Shares Western Asset Institutional Cash Reserves Class L
Management fees		0.20%
Distribution and service (12b-1) fees		0.10%
Other expenses		0.03%
Total annual fund operating expenses		0.33%
Fees waived and/or expenses reimbursed	[1]	(0.03%)
Total annual fund operating expenses after waiving fees and/or reimbursing expenses		0.30%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
L Shares Western Asset Institutional Cash Reserves Class L	31	100	180	413

The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Class L shares. The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006):1.31

Worst quarter
(09/30/2011): 0.02

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.11

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
L Shares Western Asset Institutional Cash Reserves Class L	0.12%	1.78%	2.09%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.30% for Class L shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Institutional | Western Asset Institutional AMT Free Municipal Money Market Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional AMT Free Municipal Money Market Fund
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund seeks current income that is exempt from federal income tax to the extent consistent with preservation of capital and the maintenance of liquidity.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Institutional Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2014
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which, under normal circumstances, will invest substantially all of its assets in short-term high quality municipal securities whose interest is exempt from federal income tax, including the federal alternative minimum tax. Municipal securities include debt obligations issued by any of the 50 states and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities and other structured securities such as variable rate demand obligations, tender option bonds, partnership interests and swap-based securities, many of which may be issued or backed by U.S. or non-U.S. banks. Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category (or, with respect to not more than 3% of its total assets, in the second highest category) or, if not rated, are determined by the subadviser to be of equivalent quality. The interest paid on municipal securities purchased by the fund is excluded from gross income for regular federal income tax purposes, and, as noted above, will not typically be subject to the federal alternative minimum tax. As a result, the rate of interest paid on these securities normally is lower than the rate of interest paid on fully taxable securities. Under normal circumstances, the fund must invest at least 80% of its net assets in short-term high quality municipal obligations and interests in municipal obligations that pay interest that is exempt from federal income tax, including the federal alternative minimum tax. Although the fund will ordinarily not invest in securities the interest on which is subject to regular federal income tax or the federal alternative minimum tax, when the subadviser believes that suitable AMT-free municipal securities are not available, the fund may temporarily invest up to 20% of its assets in investments that pay interest that may be subject to federal income tax or the federal alternative minimum tax. In addition, the fund may depart from its principal investment strategies in response to unusually adverse market, economic or political conditions, as described under “More on the funds’ investment strategies, investments and risks—Defensive investing.” As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term municipal debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated. Tax risk. The income on the fund’s municipal securities could become subject to federal income tax due to noncompliant conduct by issuers, unfavorable legislation or litigation or adverse interpretations by regulatory authorities. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Risks relating to investments in municipal securities. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares). The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares).
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (09/30/2007): 0.88 Worst quarter (03/31/2010): 0.02 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.04
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Institutional | Western Asset Institutional AMT Free Municipal Money Market Fund | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.25%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.06%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.31%
Fees waived and/or expenses reimbursed	rr_FeeWaiverOrReimbursementOverAssets	(0.08%)	[1]
Total annual fund operating expenses after waiving fees and/or reimbursing expenses	rr_NetExpensesOverAssets	0.23%
1 year	rr_ExpenseExampleYear01	24
3 years	rr_ExpenseExampleYear03	84
5 years	rr_ExpenseExampleYear05	158
10 years	rr_ExpenseExampleYear10	378
2002	rr_AnnualReturn2002	1.34%
2003	rr_AnnualReturn2003	0.89%
2004	rr_AnnualReturn2004	1.04%
2005	rr_AnnualReturn2005	2.23%
2006	rr_AnnualReturn2006	3.27%
2007	rr_AnnualReturn2007	3.47%
2008	rr_AnnualReturn2008	2.16%
2009	rr_AnnualReturn2009	0.38%
2010	rr_AnnualReturn2010	0.19%
2011	rr_AnnualReturn2011	0.16%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.04%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2007
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.88%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2010
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.02%
1 year	rr_AverageAnnualReturnYear01	0.16%
5 years	rr_AverageAnnualReturnYear05	1.27%
10 years	rr_AverageAnnualReturnYear10	1.51%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.23% for Institutional Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Institutional | Western Asset Institutional Government Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Government Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund seeks maximum current income to the extent consistent with preservation of capital and the maintenance of liquidity.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Institutional Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Government Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Government Portfolio.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Government Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Government Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests exclusively in short-term U.S. government obligations, including U.S. Treasuries and securities issued or guaranteed by the U.S. government or its agencies, authorities, instrumentalities or sponsored entities and in repurchase agreements. These securities may pay interest at fixed, floating or adjustable rates or may be issued at a discount. U.S. government obligations are not necessarily backed by the full faith and credit of the United States. Although the fund invests in U.S. government obligations, an investment in the fund is neither insured nor guaranteed by the U.S. government. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares). The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares).
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.28 Worst quarter (09/30/2011): 0.01 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.03
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Institutional | Western Asset Institutional Government Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.20%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.02%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.22%
1 year	rr_ExpenseExampleYear01	23
3 years	rr_ExpenseExampleYear03	72
5 years	rr_ExpenseExampleYear05	125
10 years	rr_ExpenseExampleYear10	281
2002	rr_AnnualReturn2002	1.54%
2003	rr_AnnualReturn2003	0.95%
2004	rr_AnnualReturn2004	1.16%
2005	rr_AnnualReturn2005	3.04%
2006	rr_AnnualReturn2006	4.83%
2007	rr_AnnualReturn2007	5.02%
2008	rr_AnnualReturn2008	2.46%
2009	rr_AnnualReturn2009	0.39%
2010	rr_AnnualReturn2010	0.09%
2011	rr_AnnualReturn2011	0.04%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.03%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.28%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.01%
1 year	rr_AverageAnnualReturnYear01	0.04%
5 years	rr_AverageAnnualReturnYear05	1.58%
10 years	rr_AverageAnnualReturnYear10	1.94%

XML

Administrative | Western Asset Institutional U.S. Treasury Reserves

Western Asset Institutional U.S. Treasury Reserves

Investment objective

The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income from U.S. government obligations as is consistent with preservation of capital.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Administrative Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Administrative Western Asset Institutional U.S. Treasury Reserves Administrative Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Administrative Western Asset Institutional U.S. Treasury Reserves Administrative Shares
Management fees		0.18%
Distribution and service (12b-1) fees		0.20%
Other expenses	[1]	0.02%
Total annual fund operating expenses		0.40%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Administrative Western Asset Institutional U.S. Treasury Reserves Administrative Shares	41	129	225	506

The fund invests in securities through an underlying fund: U.S. Treasury Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of U.S. Treasury Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which invests all of its assets in direct obligations of the U.S. Treasury, including U.S. Treasury bills, notes and bonds. The fund will not enter into repurchase agreements, but may enter into reverse repurchase agreements to satisfy redemption requests or for other temporary or emergency purposes. Although the fund invests in U.S. government obligations, an investment in the fund is neither insured nor guaranteed by the U.S. government.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.22

Worst quarter
(03/31/2010): 0.00

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.01

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Administrative Western Asset Institutional U.S. Treasury Reserves Institutional Shares	0.02%	1.22%	1.68%

[1] "Other expenses" for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

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Investor | Western Asset Institutional Liquid Reserves

Western Asset Institutional Liquid Reserves

Investment objective

The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Investor Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Investor Western Asset Institutional Liquid Reserves Investor Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Investor Western Asset Institutional Liquid Reserves Investor Shares
Management fees		0.21%
Distribution and service (12b-1) fees		0.10%
Other expenses	[1]	0.02%
Total annual fund operating expenses		0.33%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund's operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Investor Western Asset Institutional Liquid Reserves Investor Shares	34	106	186	418

The fund invests in securities through an underlying fund: Liquid Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Liquid Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.34

Worst quarter
(09/30/2011): 0.03

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.15

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Investor Western Asset Institutional Liquid Reserves Institutional Shares	0.17%	1.88%	2.18%

[1] "Other expenses" for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

XML

Institutional | Western Asset Institutional Tax Free Reserves

Western Asset Institutional Tax Free Reserves

Investment objective

The fund’s investment objectives are to provide shareholders with high levels of current income exempt from federal income taxes, preservation of capital and liquidity.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Institutional Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Institutional Western Asset Institutional Tax Free Reserves Institutional Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Institutional Western Asset Institutional Tax Free Reserves Institutional Shares
Management fees		0.24%
Distribution and service (12b-1) fees		none
Other expenses		0.05%
Total annual fund operating expenses		0.29%
Fees waived and/or expenses reimbursed	[1]	(0.09%)
Total annual fund operating expenses after waiving fees and/or reimbursing expenses		0.20%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Institutional Western Asset Institutional Tax Free Reserves Institutional Shares	20	74	144	349

The fund invests in securities through an underlying fund: Tax Free Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Tax Free Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which, under normal market conditions, invests at least 80% of its assets in short-term high quality municipal obligations and interests in municipal obligations that pay interest that is exempt from federal income tax, including the federal alternative minimum tax. Municipal securities include debt obligations issued by any of the 50 states and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities and other structured securities such as variable rate demand obligations, tender option bonds, partnership interests and swap-based securities, many of which may be issued or backed by U.S. or non-U.S. banks.

Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category (or, with respect to not more than 3% of its total assets, in the second highest category) or, if not rated, are determined by the subadviser to be of equivalent quality.

Under normal circumstances, the fund may invest up to 20% of its assets in investments that pay interest that may be subject to regular federal income tax and/or the federal alternative minimum tax, although for temporary or defensive purposes, the fund may invest an unlimited amount in such securities.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term municipal debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated.

Tax risk. The income on the fund’s municipal securities could become subject to federal income tax due to noncompliant conduct by issuers, unfavorable legislation or litigation or adverse interpretations by regulatory authorities.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Risks relating to investments in municipal securities. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

The fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund. However, the fund intends to comply with the diversification requirements applicable to money market funds which limit the fund’s ability to invest in the obligations of a single issuer.

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares. The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(06/30/2007): 0.90

Worst quarter
(12/31/2011): 0.02

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.05

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Institutional Western Asset Institutional Tax Free Reserves Institutional Shares	0.16%	1.27%	1.51%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.20% for Institutional Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Institutional | Western Asset Institutional Liquid Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Liquid Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Institutional Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2014
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Liquid Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Liquid Reserves Portfolio.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Liquid Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Liquid Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares). The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares).
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.34 Worst quarter (09/30/2011): 0.03 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.15
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Institutional | Western Asset Institutional Liquid Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.21%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.02%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.23%
Fees waived and/or expenses reimbursed	rr_FeeWaiverOrReimbursementOverAssets	(0.03%)	[1]
Total annual fund operating expenses after waiving fees and/or reimbursing expenses	rr_NetExpensesOverAssets	0.20%
1 year	rr_ExpenseExampleYear01	20
3 years	rr_ExpenseExampleYear03	67
5 years	rr_ExpenseExampleYear05	122
10 years	rr_ExpenseExampleYear10	286
2002	rr_AnnualReturn2002	1.85%
2003	rr_AnnualReturn2003	1.10%
2004	rr_AnnualReturn2004	1.31%
2005	rr_AnnualReturn2005	3.20%
2006	rr_AnnualReturn2006	4.99%
2007	rr_AnnualReturn2007	5.32%
2008	rr_AnnualReturn2008	3.11%
2009	rr_AnnualReturn2009	0.68%
2010	rr_AnnualReturn2010	0.23%
2011	rr_AnnualReturn2011	0.17%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.15%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.34%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.03%
1 year	rr_AverageAnnualReturnYear01	0.17%
5 years	rr_AverageAnnualReturnYear05	1.88%
10 years	rr_AverageAnnualReturnYear10	2.18%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.20% for Institutional Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Premium | Western Asset Institutional AMT Free Municipal Money Market Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional AMT Free Municipal Money Market Fund
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund seeks current income that is exempt from federal income tax to the extent consistent with preservation of capital and the maintenance of liquidity.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Premium Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other expenses” for Premium Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which, under normal circumstances, will invest substantially all of its assets in short-term high quality municipal securities whose interest is exempt from federal income tax, including the federal alternative minimum tax. Municipal securities include debt obligations issued by any of the 50 states and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities and other structured securities such as variable rate demand obligations, tender option bonds, partnership interests and swap-based securities, many of which may be issued or backed by U.S. or non-U.S. banks. Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category (or, with respect to not more than 3% of its total assets, in the second highest category) or, if not rated, are determined by the subadviser to be of equivalent quality. The interest paid on municipal securities purchased by the fund is excluded from gross income for regular federal income tax purposes, and, as noted above, will not typically be subject to the federal alternative minimum tax. As a result, the rate of interest paid on these securities normally is lower than the rate of interest paid on fully taxable securities. Under normal circumstances, the fund must invest at least 80% of its net assets in short-term high quality municipal obligations and interests in municipal obligations that pay interest that is exempt from federal income tax, including the federal alternative minimum tax. Although the fund will ordinarily not invest in securities the interest on which is subject to regular federal income tax or the federal alternative minimum tax, when the subadviser believes that suitable AMT-free municipal securities are not available, the fund may temporarily invest up to 20% of its assets in investments that pay interest that may be subject to federal income tax or the federal alternative minimum tax. In addition, the fund may depart from its principal investment strategies in response to unusually adverse market, economic or political conditions, as described under “More on the funds’ investment strategies, investments and risks—Defensive investing.” As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term municipal debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated. Tax risk. The income on the fund’s municipal securities could become subject to federal income tax due to noncompliant conduct by issuers, unfavorable legislation or litigation or adverse interpretations by regulatory authorities. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Risks relating to investments in municipal securities. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Premium Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Premium Shares because there were no Premium Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Premium Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (09/30/2007): 0.88 Worst quarter (03/31/2010): 0.02 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.04
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Premium Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Premium Shares because there were no Premium Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ.
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Premium | Western Asset Institutional AMT Free Municipal Money Market Fund | Premium Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.25%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.10%
Other expenses	rr_OtherExpensesOverAssets	0.06%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.41%
1 year	rr_ExpenseExampleYear01	42
3 years	rr_ExpenseExampleYear03	132
5 years	rr_ExpenseExampleYear05	230
10 years	rr_ExpenseExampleYear10	517
Premium | Western Asset Institutional AMT Free Municipal Money Market Fund | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
2002	rr_AnnualReturn2002	1.34%
2003	rr_AnnualReturn2003	0.89%
2004	rr_AnnualReturn2004	1.04%
2005	rr_AnnualReturn2005	2.23%
2006	rr_AnnualReturn2006	3.27%
2007	rr_AnnualReturn2007	3.47%
2008	rr_AnnualReturn2008	2.16%
2009	rr_AnnualReturn2009	0.38%
2010	rr_AnnualReturn2010	0.19%
2011	rr_AnnualReturn2011	0.16%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.04%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2007
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.88%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2010
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.02%
1 year	rr_AverageAnnualReturnYear01	0.16%
5 years	rr_AverageAnnualReturnYear05	1.27%
10 years	rr_AverageAnnualReturnYear10	1.51%

[1] "Other expenses" for Premium Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

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Administrative | Western Asset Institutional Tax Free Reserves

Western Asset Institutional Tax Free Reserves

Investment objective

The fund’s investment objectives are to provide shareholders with high levels of current income exempt from federal income taxes, preservation of capital and liquidity.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Administrative Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Administrative Western Asset Institutional Tax Free Reserves Administrative Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Administrative Western Asset Institutional Tax Free Reserves Administrative Shares
Management fees		0.24%
Distribution and service (12b-1) fees		0.20%
Other expenses	[1]	0.05%
Total annual fund operating expenses		0.49%
Fees waived and/or expenses reimbursed	[2]	(0.09%)
Total annual fund operating expenses after waiving fees and/or reimbursing expenses		0.40%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Administrative Western Asset Institutional Tax Free Reserves Administrative Shares	41	139	256	598

The fund invests in securities through an underlying fund: Tax Free Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Tax Free Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which, under normal market conditions, invests at least 80% of its assets in short-term high quality municipal obligations and interests in municipal obligations that pay interest that is exempt from federal income tax, including the federal alternative minimum tax. Municipal securities include debt obligations issued by any of the 50 states and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities and other structured securities such as variable rate demand obligations, tender option bonds, partnership interests and swap-based securities, many of which may be issued or backed by U.S. or non-U.S. banks.

Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category (or, with respect to not more than 3% of its total assets, in the second highest category) or, if not rated, are determined by the subadviser to be of equivalent quality.

Under normal circumstances, the fund may invest up to 20% of its assets in investments that pay interest that may be subject to regular federal income tax and/or the federal alternative minimum tax, although for temporary or defensive purposes, the fund may invest an unlimited amount in such securities.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term municipal debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated.

Tax risk. The income on the fund’s municipal securities could become subject to federal income tax due to noncompliant conduct by issuers, unfavorable legislation or litigation or adverse interpretations by regulatory authorities.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Risks relating to investments in municipal securities. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

The fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund. However, the fund intends to comply with the diversification requirements applicable to money market funds which limit the fund’s ability to invest in the obligations of a single issuer.

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(06/30/2007): 0.90

Worst quarter
(12/31/2011): 0.02

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.05

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Administrative Western Asset Institutional Tax Free Reserves Institutional Shares	0.16%	1.27%	1.51%

[1] "Other expenses" for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

[2] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.40% for Administrative Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

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Investor | Western Asset Institutional Tax Free Reserves

Western Asset Institutional Tax Free Reserves

Investment objective

The fund’s investment objectives are to provide shareholders with high levels of current income exempt from federal income taxes, preservation of capital and liquidity.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Investor Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Investor Western Asset Institutional Tax Free Reserves Investor Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Investor Western Asset Institutional Tax Free Reserves Investor Shares
Management fees		0.24%
Distribution and service (12b-1) fees		0.10%
Other expenses	[1]	0.05%
Total annual fund operating expenses		0.39%
Fees waived and/or expenses reimbursed	[2]	(0.04%)
Total annual fund operating expenses after waiving fees and/or reimbursing expenses		0.35%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Investor Western Asset Institutional Tax Free Reserves Investor Shares	36	117	211	485

The fund invests in securities through an underlying fund: Tax Free Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Tax Free Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which, under normal market conditions, invests at least 80% of its assets in short-term high quality municipal obligations and interests in municipal obligations that pay interest that is exempt from federal income tax, including the federal alternative minimum tax. Municipal securities include debt obligations issued by any of the 50 states and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities and other structured securities such as variable rate demand obligations, tender option bonds, partnership interests and swap-based securities, many of which may be issued or backed by U.S. or non-U.S. banks.

Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category (or, with respect to not more than 3% of its total assets, in the second highest category) or, if not rated, are determined by the subadviser to be of equivalent quality.

Under normal circumstances, the fund may invest up to 20% of its assets in investments that pay interest that may be subject to regular federal income tax and/or the federal alternative minimum tax, although for temporary or defensive purposes, the fund may invest an unlimited amount in such securities.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term municipal debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated.

Tax risk. The income on the fund’s municipal securities could become subject to federal income tax due to noncompliant conduct by issuers, unfavorable legislation or litigation or adverse interpretations by regulatory authorities.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Risks relating to investments in municipal securities. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

The fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund. However, the fund intends to comply with the diversification requirements applicable to money market funds which limit the fund’s ability to invest in the obligations of a single issuer.

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(06/30/2007): 0.90

Worst quarter
(12/31/2011): 0.02

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.05

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Investor Western Asset Institutional Tax Free Reserves Institutional Shares	0.16%	1.27%	1.51%

[1] "Other expenses" for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

[2] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.35% for Investor Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees’ consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class’ total annual operating expenses have fallen to a level below the limit described above.

XML

Institutional | Western Asset Institutional Liquid Reserves

Western Asset Institutional Liquid Reserves

Investment objective

The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Institutional Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Institutional Western Asset Institutional Liquid Reserves Institutional Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Institutional Western Asset Institutional Liquid Reserves Institutional Shares
Management fees		0.21%
Distribution and service (12b-1) fees		none
Other expenses		0.02%
Total annual fund operating expenses		0.23%
Fees waived and/or expenses reimbursed	[1]	(0.03%)
Total annual fund operating expenses after waiving fees and/or reimbursing expenses		0.20%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Institutional Western Asset Institutional Liquid Reserves Institutional Shares	20	67	122	286

The fund invests in securities through an underlying fund: Liquid Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Liquid Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares). The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.34

Worst quarter
(09/30/2011): 0.03

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.15

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Institutional Western Asset Institutional Liquid Reserves Institutional Shares	0.17%	1.88%	2.18%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.20% for Institutional Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Premium | Western Asset Institutional Cash Reserves

Western Asset Institutional Cash Reserves

Investment objective

The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Premium Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Premium Western Asset Institutional Cash Reserves Premium Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Premium Western Asset Institutional Cash Reserves Premium Shares
Management fees		0.20%
Distribution and service (12b-1) fees		0.10%
Other expenses	[1]	0.03%
Total annual fund operating expenses		0.33%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Premium Western Asset Institutional Cash Reserves Premium Shares	34	106	186	418

The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Class L shares, a class of the fund that is invested in the same portfolio of securities as Premium Shares. The table shows the average annual total returns of Class L shares. No performance information is presented for Premium Shares because there were no Premium Shares outstanding during the periods shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.31

Worst quarter
(09/30/2011): 0.02

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.11

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Premium Western Asset Institutional Cash Reserves Class L	0.12%	1.78%	2.09%

[1] "Other expenses" for Premium Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Administrative | Western Asset Institutional Government Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Government Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund seeks maximum current income to the extent consistent with preservation of capital and the maintenance of liquidity.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Administrative Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2014
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Government Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Government Portfolio.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other expenses” for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Government Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Government Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests exclusively in short-term U.S. government obligations, including U.S. Treasuries and securities issued or guaranteed by the U.S. government or its agencies, authorities, instrumentalities or sponsored entities and in repurchase agreements. These securities may pay interest at fixed, floating or adjustable rates or may be issued at a discount. U.S. government obligations are not necessarily backed by the full faith and credit of the United States. Although the fund invests in U.S. government obligations, an investment in the fund is neither insured nor guaranteed by the U.S. government. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.28 Worst quarter (09/30/2011): 0.01 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.03
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ.
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Administrative | Western Asset Institutional Government Reserves | Administrative Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.20%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.20%
Other expenses	rr_OtherExpensesOverAssets	0.02%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.42%
Fees waived and/or expenses reimbursed	rr_FeeWaiverOrReimbursementOverAssets	(0.02%)	[2]
Total annual fund operating expenses after waiving fees and/or reimbursing expenses	rr_NetExpensesOverAssets	0.40%
1 year	rr_ExpenseExampleYear01	41
3 years	rr_ExpenseExampleYear03	133
5 years	rr_ExpenseExampleYear05	231
10 years	rr_ExpenseExampleYear10	525
Administrative | Western Asset Institutional Government Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
2002	rr_AnnualReturn2002	1.54%
2003	rr_AnnualReturn2003	0.95%
2004	rr_AnnualReturn2004	1.16%
2005	rr_AnnualReturn2005	3.04%
2006	rr_AnnualReturn2006	4.83%
2007	rr_AnnualReturn2007	5.02%
2008	rr_AnnualReturn2008	2.46%
2009	rr_AnnualReturn2009	0.39%
2010	rr_AnnualReturn2010	0.09%
2011	rr_AnnualReturn2011	0.04%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.03%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.28%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.01%
1 year	rr_AverageAnnualReturnYear01	0.04%
5 years	rr_AverageAnnualReturnYear05	1.58%
10 years	rr_AverageAnnualReturnYear10	1.94%

[1] "Other expenses" for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

[2] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.40% for Administrative Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Administrative | Western Asset Institutional Cash Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Cash Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Administrative Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2014
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other expenses” for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class O shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class O shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.32 Worst quarter (09/30/2011): 0.03 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.15
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class O shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ.
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Administrative | Western Asset Institutional Cash Reserves | Administrative Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.20%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.20%
Other expenses	rr_OtherExpensesOverAssets	0.03%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.43%
Fees waived and/or expenses reimbursed	rr_FeeWaiverOrReimbursementOverAssets	(0.03%)	[2]
Total annual fund operating expenses after waiving fees and/or reimbursing expenses	rr_NetExpensesOverAssets	0.40%
1 year	rr_ExpenseExampleYear01	41
3 years	rr_ExpenseExampleYear03	132
5 years	rr_ExpenseExampleYear05	235
10 years	rr_ExpenseExampleYear10	537
Administrative | Western Asset Institutional Cash Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
2003	rr_AnnualReturn2003	1.08%
2004	rr_AnnualReturn2004	1.30%
2005	rr_AnnualReturn2005	3.19%
2006	rr_AnnualReturn2006	5.02%
2007	rr_AnnualReturn2007	5.27%
2008	rr_AnnualReturn2008	2.98%
2009	rr_AnnualReturn2009	0.62%
2010	rr_AnnualReturn2010	0.21%
2011	rr_AnnualReturn2011	0.17%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.15%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.32%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.03%
1 year	rr_AverageAnnualReturnYear01	0.17%
5 years	rr_AverageAnnualReturnYear05	1.83%
Since inception	rr_AverageAnnualReturnSinceInception	2.17%
Inception date	rr_AverageAnnualReturnInceptionDate	Oct. 02, 2002

[1] "Other expenses" for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

[2] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.40% for Administrative Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Investor | Western Asset Institutional Government Reserves

Western Asset Institutional Government Reserves

Investment objective

The fund seeks maximum current income to the extent consistent with preservation of capital and the maintenance of liquidity.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Investor Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Investor Western Asset Institutional Government Reserves Investor Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Investor Western Asset Institutional Government Reserves Investor Shares
Management fees		0.20%
Distribution and service (12b-1) fees		0.10%
Other expenses	[1]	0.02%
Total annual fund operating expenses		0.32%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Investor Western Asset Institutional Government Reserves Investor Shares	33	103	180	405

The fund invests in securities through an underlying fund: Government Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Government Portfolio.

Principal investment strategies

The fund is a money market fund which invests exclusively in short-term U.S. government obligations, including U.S. Treasuries and securities issued or guaranteed by the U.S. government or its agencies, authorities, instrumentalities or sponsored entities and in repurchase agreements. These securities may pay interest at fixed, floating or adjustable rates or may be issued at a discount. U.S. government obligations are not necessarily backed by the full faith and credit of the United States. Although the fund invests in U.S. government obligations, an investment in the fund is neither insured nor guaranteed by the U.S. government.

The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.28

Worst quarter
(09/30/2011): 0.01

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.03

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Investor Western Asset Institutional Government Reserves Institutional Shares	0.04%	1.58%	1.94%

[1] "Other expenses" for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Institutional | Western Asset Institutional U.S. Treasury Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional U.S. Treasury Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income from U.S. government obligations as is consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Institutional Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: U.S. Treasury Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of U.S. Treasury Reserves Portfolio.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: U.S. Treasury Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of U.S. Treasury Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests all of its assets in direct obligations of the U.S. Treasury, including U.S. Treasury bills, notes and bonds. The fund will not enter into repurchase agreements, but may enter into reverse repurchase agreements to satisfy redemption requests or for other temporary or emergency purposes. Although the fund invests in U.S. government obligations, an investment in the fund is neither insured nor guaranteed by the U.S. government. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares. The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.22 Worst quarter (03/31/2010): 0.00 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.01
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Institutional | Western Asset Institutional U.S. Treasury Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.18%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.02%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.20%
1 year	rr_ExpenseExampleYear01	20
3 years	rr_ExpenseExampleYear03	63
5 years	rr_ExpenseExampleYear05	112
10 years	rr_ExpenseExampleYear10	254
2002	rr_AnnualReturn2002	1.52%
2003	rr_AnnualReturn2003	0.83%
2004	rr_AnnualReturn2004	1.02%
2005	rr_AnnualReturn2005	2.79%
2006	rr_AnnualReturn2006	4.59%
2007	rr_AnnualReturn2007	4.42%
2008	rr_AnnualReturn2008	1.58%
2009	rr_AnnualReturn2009	0.13%
2010	rr_AnnualReturn2010	0.02%
2011	rr_AnnualReturn2011	0.02%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.01%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.22%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2010
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.00%
1 year	rr_AverageAnnualReturnYear01	0.02%
5 years	rr_AverageAnnualReturnYear05	1.22%
10 years	rr_AverageAnnualReturnYear10	1.68%

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Institutional | Western Asset Institutional Tax Free Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Tax Free Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objectives are to provide shareholders with high levels of current income exempt from federal income taxes, preservation of capital and liquidity.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Institutional Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2014
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Tax Free Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Tax Free Reserves Portfolio.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Tax Free Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Tax Free Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which, under normal market conditions, invests at least 80% of its assets in short-term high quality municipal obligations and interests in municipal obligations that pay interest that is exempt from federal income tax, including the federal alternative minimum tax. Municipal securities include debt obligations issued by any of the 50 states and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities and other structured securities such as variable rate demand obligations, tender option bonds, partnership interests and swap-based securities, many of which may be issued or backed by U.S. or non-U.S. banks. Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category (or, with respect to not more than 3% of its total assets, in the second highest category) or, if not rated, are determined by the subadviser to be of equivalent quality. Under normal circumstances, the fund may invest up to 20% of its assets in investments that pay interest that may be subject to regular federal income tax and/or the federal alternative minimum tax, although for temporary or defensive purposes, the fund may invest an unlimited amount in such securities. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term municipal debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated. Tax risk. The income on the fund’s municipal securities could become subject to federal income tax due to noncompliant conduct by issuers, unfavorable legislation or litigation or adverse interpretations by regulatory authorities. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Risks relating to investments in municipal securities. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”). The fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund. However, the fund intends to comply with the diversification requirements applicable to money market funds which limit the fund’s ability to invest in the obligations of a single issuer.
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Nondiversified Status [Text]	rr_RiskNondiversifiedStatus	The fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund. However, the fund intends to comply with the diversification requirements applicable to money market funds which limit the fund’s ability to invest in the obligations of a single issuer.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares. The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (06/30/2007): 0.90 Worst quarter (12/31/2011): 0.02 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.05
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Institutional | Western Asset Institutional Tax Free Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.24%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.05%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.29%
Fees waived and/or expenses reimbursed	rr_FeeWaiverOrReimbursementOverAssets	(0.09%)	[1]
Total annual fund operating expenses after waiving fees and/or reimbursing expenses	rr_NetExpensesOverAssets	0.20%
1 year	rr_ExpenseExampleYear01	20
3 years	rr_ExpenseExampleYear03	74
5 years	rr_ExpenseExampleYear05	144
10 years	rr_ExpenseExampleYear10	349
2002	rr_AnnualReturn2002	1.40%
2003	rr_AnnualReturn2003	0.89%
2004	rr_AnnualReturn2004	1.02%
2005	rr_AnnualReturn2005	2.25%
2006	rr_AnnualReturn2006	3.27%
2007	rr_AnnualReturn2007	3.50%
2008	rr_AnnualReturn2008	2.13%
2009	rr_AnnualReturn2009	0.41%
2010	rr_AnnualReturn2010	0.18%
2011	rr_AnnualReturn2011	0.16%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.05%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2007
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.90%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.02%
1 year	rr_AverageAnnualReturnYear01	0.16%
5 years	rr_AverageAnnualReturnYear05	1.27%
10 years	rr_AverageAnnualReturnYear10	1.51%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.20% for Institutional Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Institutional Liquid Reserves Shares | Western Asset Institutional Liquid Reserves

Western Asset Institutional Liquid Reserves

Investment objective

The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold SVB Securities Institutional Liquid Reserves Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Institutional Liquid Reserves Shares Western Asset Institutional Liquid Reserves SVB Securities Institutional Liquid Reserves Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Institutional Liquid Reserves Shares Western Asset Institutional Liquid Reserves SVB Securities Institutional Liquid Reserves Shares
Management fees		0.21%
Distribution and service (12b-1) fees		0.10%
Other expenses		0.02%
Total annual fund operating expenses		0.33%
Fees waived and/or expenses reimbursed	[1]	(0.09%)
Total annual fund operating expenses after waiving fees and/or reimbursing expenses		0.24%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Institutional Liquid Reserves Shares Western Asset Institutional Liquid Reserves SVB Securities Institutional Liquid Reserves Shares	25	88	168	402

The fund invests in securities through an underlying fund: Liquid Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Liquid Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for SVB Securities Institutional Liquid Reserves Shares. The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available by calling your Service Agent at 1-800-303-7371.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.31

Worst quarter
(09/30/2011): 0.01

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.09

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Institutional Liquid Reserves Shares Western Asset Institutional Liquid Reserves SVB Securities Institutional Liquid Reserves Shares	0.09%	1.79%	2.10%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.24% for SVB Securities Institutional Liquid Reserves Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Administrative | Western Asset Institutional Cash Reserves

Western Asset Institutional Cash Reserves

Investment objective

The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Administrative Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Administrative Western Asset Institutional Cash Reserves Administrative Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Administrative Western Asset Institutional Cash Reserves Administrative Shares
Management fees		0.20%
Distribution and service (12b-1) fees		0.20%
Other expenses	[1]	0.03%
Total annual fund operating expenses		0.43%
Fees waived and/or expenses reimbursed	[2]	(0.03%)
Total annual fund operating expenses after waiving fees and/or reimbursing expenses		0.40%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Administrative Western Asset Institutional Cash Reserves Administrative Shares	41	132	235	537

The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class O shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.32

Worst quarter
(09/30/2011): 0.03

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.15

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	Since inception	Inception date
Administrative Western Asset Institutional Cash Reserves Institutional Shares	0.17%	1.83%	2.17%	Oct. 02, 2002

[1] "Other expenses" for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

[2] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.40% for Administrative Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Liquid Reserves Shares | Western Asset Institutional Liquid Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Liquid Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold SVB Securities Liquid Reserves Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2014
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Liquid Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Liquid Reserves Portfolio.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Liquid Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Liquid Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for SVB Securities Liquid Reserves Shares. The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available by calling your Service Agent at 1-800-303-7371. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for SVB Securities Liquid Reserves Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-800-303-7371
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.22 Worst quarter (03/31/2011): 0.00 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.01
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Liquid Reserves Shares | Western Asset Institutional Liquid Reserves | SVB Securities Liquid Reserves Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.21%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.45%
Other expenses	rr_OtherExpensesOverAssets	0.04%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.70%
Fees waived and/or expenses reimbursed	rr_FeeWaiverOrReimbursementOverAssets	(0.10%)	[1]
Total annual fund operating expenses after waiving fees and/or reimbursing expenses	rr_NetExpensesOverAssets	0.60%
1 year	rr_ExpenseExampleYear01	61
3 years	rr_ExpenseExampleYear03	203
5 years	rr_ExpenseExampleYear05	369
10 years	rr_ExpenseExampleYear10	850
2002	rr_AnnualReturn2002	1.44%
2003	rr_AnnualReturn2003	0.70%
2004	rr_AnnualReturn2004	0.89%
2005	rr_AnnualReturn2005	2.75%
2006	rr_AnnualReturn2006	4.52%
2007	rr_AnnualReturn2007	4.85%
2008	rr_AnnualReturn2008	2.66%
2009	rr_AnnualReturn2009	0.34%
2010	rr_AnnualReturn2010	0.01%
2011	rr_AnnualReturn2011	0.01%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.01%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.22%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.00%
1 year	rr_AverageAnnualReturnYear01	0.01%
5 years	rr_AverageAnnualReturnYear05	1.56%
10 years	rr_AverageAnnualReturnYear10	1.80%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.60% for SVB Securities Liquid Reserves Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Document and Entity Information	12 Months Ended
	Dec. 28, 2012
Risk/Return:
Document Type	485BPOS
Document Period End Date	Aug. 31, 2012
Registrant Name	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Central Index Key	0000889512
Amendment Flag	false
Document Creation Date	Dec. 13, 2012
Document Effective Date	Dec. 28, 2012
Prospectus Date	Dec. 28, 2012

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Document Creation Date	dei_DocumentCreationDate	Dec. 13, 2012

XML

Institutional | Western Asset Institutional Government Reserves

Western Asset Institutional Government Reserves

Investment objective

The fund seeks maximum current income to the extent consistent with preservation of capital and the maintenance of liquidity.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Institutional Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Institutional Western Asset Institutional Government Reserves Institutional Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses	Institutional Western Asset Institutional Government Reserves Institutional Shares
Management fees	0.20%
Distribution and service (12b-1) fees	none
Other expenses	0.02%
Total annual fund operating expenses	0.22%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Institutional Western Asset Institutional Government Reserves Institutional Shares	23	72	125	281

The fund invests in securities through an underlying fund: Government Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Government Portfolio.

Principal investment strategies

The fund is a money market fund which invests exclusively in short-term U.S. government obligations, including U.S. Treasuries and securities issued or guaranteed by the U.S. government or its agencies, authorities, instrumentalities or sponsored entities and in repurchase agreements. These securities may pay interest at fixed, floating or adjustable rates or may be issued at a discount. U.S. government obligations are not necessarily backed by the full faith and credit of the United States. Although the fund invests in U.S. government obligations, an investment in the fund is neither insured nor guaranteed by the U.S. government.

The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares). The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.28

Worst quarter
(09/30/2011): 0.01

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.03

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Institutional Western Asset Institutional Government Reserves Institutional Shares	0.04%	1.58%	1.94%

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Institutional | Western Asset Institutional Cash Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Cash Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Institutional Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class O shares). The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class O shares).
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.32 Worst quarter (09/30/2011): 0.03 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.15
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Institutional | Western Asset Institutional Cash Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.20%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.02%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.22%
1 year	rr_ExpenseExampleYear01	23
3 years	rr_ExpenseExampleYear03	72
5 years	rr_ExpenseExampleYear05	125
10 years	rr_ExpenseExampleYear10	281
2003	rr_AnnualReturn2003	1.08%
2004	rr_AnnualReturn2004	1.30%
2005	rr_AnnualReturn2005	3.19%
2006	rr_AnnualReturn2006	5.02%
2007	rr_AnnualReturn2007	5.27%
2008	rr_AnnualReturn2008	2.98%
2009	rr_AnnualReturn2009	0.62%
2010	rr_AnnualReturn2010	0.21%
2011	rr_AnnualReturn2011	0.17%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.15%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.32%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.03%
1 year	rr_AverageAnnualReturnYear01	0.17%
5 years	rr_AverageAnnualReturnYear05	1.83%
Since Inception	rr_AverageAnnualReturnSinceInception	2.17%
Inception date	rr_AverageAnnualReturnInceptionDate	Oct. 02, 2002

XML

Institutional | Western Asset Institutional U.S. Treasury Reserves

Western Asset Institutional U.S. Treasury Reserves

Investment objective

The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income from U.S. government obligations as is consistent with preservation of capital.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Institutional Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Institutional Western Asset Institutional U.S. Treasury Reserves Institutional Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses	Institutional Western Asset Institutional U.S. Treasury Reserves Institutional Shares
Management fees	0.18%
Distribution and service (12b-1) fees	none
Other expenses	0.02%
Total annual fund operating expenses	0.20%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Institutional Western Asset Institutional U.S. Treasury Reserves Institutional Shares	20	63	112	254

The fund invests in securities through an underlying fund: U.S. Treasury Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of U.S. Treasury Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which invests all of its assets in direct obligations of the U.S. Treasury, including U.S. Treasury bills, notes and bonds. The fund will not enter into repurchase agreements, but may enter into reverse repurchase agreements to satisfy redemption requests or for other temporary or emergency purposes. Although the fund invests in U.S. government obligations, an investment in the fund is neither insured nor guaranteed by the U.S. government.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares. The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.22

Worst quarter
(03/31/2010): 0.00

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.01

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Institutional Western Asset Institutional U.S. Treasury Reserves Institutional Shares	0.02%	1.22%	1.68%

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Investor | Western Asset Institutional U.S. Treasury Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional U.S. Treasury Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income from U.S. government obligations as is consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Investor Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: U.S. Treasury Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of U.S. Treasury Reserves Portfolio.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other expenses” for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: U.S. Treasury Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of U.S. Treasury Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests all of its assets in direct obligations of the U.S. Treasury, including U.S. Treasury bills, notes and bonds. The fund will not enter into repurchase agreements, but may enter into reverse repurchase agreements to satisfy redemption requests or for other temporary or emergency purposes. Although the fund invests in U.S. government obligations, an investment in the fund is neither insured nor guaranteed by the U.S. government. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Investor Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.22 Worst quarter (03/31/2010): 0.00 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.01
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ.
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Investor | Western Asset Institutional U.S. Treasury Reserves | Investor Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.18%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.10%
Other expenses	rr_OtherExpensesOverAssets	0.02%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.30%
1 year	rr_ExpenseExampleYear01	31
3 years	rr_ExpenseExampleYear03	97
5 years	rr_ExpenseExampleYear05	169
10 years	rr_ExpenseExampleYear10	380
Investor | Western Asset Institutional U.S. Treasury Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
2002	rr_AnnualReturn2002	1.52%
2003	rr_AnnualReturn2003	0.83%
2004	rr_AnnualReturn2004	1.02%
2005	rr_AnnualReturn2005	2.79%
2006	rr_AnnualReturn2006	4.59%
2007	rr_AnnualReturn2007	4.42%
2008	rr_AnnualReturn2008	1.58%
2009	rr_AnnualReturn2009	0.13%
2010	rr_AnnualReturn2010	0.02%
2011	rr_AnnualReturn2011	0.02%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.01%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.22%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2010
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.00%
1 year	rr_AverageAnnualReturnYear01	0.02%
5 years	rr_AverageAnnualReturnYear05	1.22%
10 years	rr_AverageAnnualReturnYear10	1.68%

[1] "Other expenses" for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Administrative | Western Asset Institutional Tax Free Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Tax Free Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objectives are to provide shareholders with high levels of current income exempt from federal income taxes, preservation of capital and liquidity.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Administrative Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2014
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Tax Free Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Tax Free Reserves Portfolio.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other expenses” for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Tax Free Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Tax Free Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which, under normal market conditions, invests at least 80% of its assets in short-term high quality municipal obligations and interests in municipal obligations that pay interest that is exempt from federal income tax, including the federal alternative minimum tax. Municipal securities include debt obligations issued by any of the 50 states and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities and other structured securities such as variable rate demand obligations, tender option bonds, partnership interests and swap-based securities, many of which may be issued or backed by U.S. or non-U.S. banks. Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category (or, with respect to not more than 3% of its total assets, in the second highest category) or, if not rated, are determined by the subadviser to be of equivalent quality. Under normal circumstances, the fund may invest up to 20% of its assets in investments that pay interest that may be subject to regular federal income tax and/or the federal alternative minimum tax, although for temporary or defensive purposes, the fund may invest an unlimited amount in such securities. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term municipal debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated. Tax risk. The income on the fund’s municipal securities could become subject to federal income tax due to noncompliant conduct by issuers, unfavorable legislation or litigation or adverse interpretations by regulatory authorities. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Risks relating to investments in municipal securities. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”). The fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund. However, the fund intends to comply with the diversification requirements applicable to money market funds which limit the fund’s ability to invest in the obligations of a single issuer.
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Nondiversified Status [Text]	rr_RiskNondiversifiedStatus	The fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund. However, the fund intends to comply with the diversification requirements applicable to money market funds which limit the fund’s ability to invest in the obligations of a single issuer.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Administrative Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (06/30/2007): 0.90 Worst quarter (12/31/2011): 0.02 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.05
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ.
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Administrative | Western Asset Institutional Tax Free Reserves | Administrative Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.24%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.20%
Other expenses	rr_OtherExpensesOverAssets	0.05%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.49%
Fees waived and/or expenses reimbursed	rr_FeeWaiverOrReimbursementOverAssets	(0.09%)	[2]
Total annual fund operating expenses after waiving fees and/or reimbursing expenses	rr_NetExpensesOverAssets	0.40%
1 year	rr_ExpenseExampleYear01	41
3 years	rr_ExpenseExampleYear03	139
5 years	rr_ExpenseExampleYear05	256
10 years	rr_ExpenseExampleYear10	598
Administrative | Western Asset Institutional Tax Free Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
2002	rr_AnnualReturn2002	1.40%
2003	rr_AnnualReturn2003	0.89%
2004	rr_AnnualReturn2004	1.02%
2005	rr_AnnualReturn2005	2.25%
2006	rr_AnnualReturn2006	3.27%
2007	rr_AnnualReturn2007	3.50%
2008	rr_AnnualReturn2008	2.13%
2009	rr_AnnualReturn2009	0.41%
2010	rr_AnnualReturn2010	0.18%
2011	rr_AnnualReturn2011	0.16%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.05%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2007
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.90%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.02%
1 year	rr_AverageAnnualReturnYear01	0.16%
5 years	rr_AverageAnnualReturnYear05	1.27%
10 years	rr_AverageAnnualReturnYear10	1.51%

[1] "Other expenses" for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

[2] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.40% for Administrative Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Investor | Western Asset Institutional AMT Free Municipal Money Market Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional AMT Free Municipal Money Market Fund
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund seeks current income that is exempt from federal income tax to the extent consistent with preservation of capital and the maintenance of liquidity.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Investor Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2014
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other expenses” for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which, under normal circumstances, will invest substantially all of its assets in short-term high quality municipal securities whose interest is exempt from federal income tax, including the federal alternative minimum tax. Municipal securities include debt obligations issued by any of the 50 states and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities and other structured securities such as variable rate demand obligations, tender option bonds, partnership interests and swap-based securities, many of which may be issued or backed by U.S. or non-U.S. banks. Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself. The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category (or, with respect to not more than 3% of its total assets, in the second highest category) or, if not rated, are determined by the subadviser to be of equivalent quality. The interest paid on municipal securities purchased by the fund is excluded from gross income for regular federal income tax purposes, and, as noted above, will not typically be subject to the federal alternative minimum tax. As a result, the rate of interest paid on these securities normally is lower than the rate of interest paid on fully taxable securities. Under normal circumstances, the fund must invest at least 80% of its net assets in short-term high quality municipal obligations and interests in municipal obligations that pay interest that is exempt from federal income tax, including the federal alternative minimum tax. Although the fund will ordinarily not invest in securities the interest on which is subject to regular federal income tax or the federal alternative minimum tax, when the subadviser believes that suitable AMT-free municipal securities are not available, the fund may temporarily invest up to 20% of its assets in investments that pay interest that may be subject to federal income tax or the federal alternative minimum tax. In addition, the fund may depart from its principal investment strategies in response to unusually adverse market, economic or political conditions, as described under “More on the funds’ investment strategies, investments and risks—Defensive investing.” As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term municipal debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated. Tax risk. The income on the fund’s municipal securities could become subject to federal income tax due to noncompliant conduct by issuers, unfavorable legislation or litigation or adverse interpretations by regulatory authorities. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in municipal securities and interests in municipal securities that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Risks relating to investments in municipal securities. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (09/30/2007): 0.88 Worst quarter (03/31/2010): 0.02 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.04
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ.
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Investor | Western Asset Institutional AMT Free Municipal Money Market Fund | Investor Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.25%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.10%
Other expenses	rr_OtherExpensesOverAssets	0.06%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.41%
Fees waived and/or expenses reimbursed	rr_FeeWaiverOrReimbursementOverAssets	(0.06%)	[2]
Total annual fund operating expenses after waiving fees and/or reimbursing expenses	rr_NetExpensesOverAssets	0.35%
1 year	rr_ExpenseExampleYear01	36
3 years	rr_ExpenseExampleYear03	119
5 years	rr_ExpenseExampleYear05	217
10 years	rr_ExpenseExampleYear10	505
Investor | Western Asset Institutional AMT Free Municipal Money Market Fund | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
2002	rr_AnnualReturn2002	1.34%
2003	rr_AnnualReturn2003	0.89%
2004	rr_AnnualReturn2004	1.04%
2005	rr_AnnualReturn2005	2.23%
2006	rr_AnnualReturn2006	3.27%
2007	rr_AnnualReturn2007	3.47%
2008	rr_AnnualReturn2008	2.16%
2009	rr_AnnualReturn2009	0.38%
2010	rr_AnnualReturn2010	0.19%
2011	rr_AnnualReturn2011	0.16%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.04%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2007
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.88%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2010
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.02%
1 year	rr_AverageAnnualReturnYear01	0.16%
5 years	rr_AverageAnnualReturnYear05	1.27%
10 years	rr_AverageAnnualReturnYear10	1.51%

[1] "Other expenses" for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

[2] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.35% for Investor Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees’ consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class’ total annual operating expenses have fallen to a level below the limit described above.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Administrative | Western Asset Institutional Liquid Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional Liquid Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Administrative Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2014
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: Liquid Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Liquid Reserves Portfolio.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other expenses” for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund's operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: Liquid Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Liquid Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality. The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Strategy Portfolio Concentration [Text]	rr_StrategyPortfolioConcentration	Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated. Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund. Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry. Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities. Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security. Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.34 Worst quarter (09/30/2011): 0.03 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.15
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ.
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Administrative | Western Asset Institutional Liquid Reserves | Administrative Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.21%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.20%
Other expenses	rr_OtherExpensesOverAssets	0.02%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.43%
Fees waived and/or expenses reimbursed	rr_FeeWaiverOrReimbursementOverAssets	(0.03%)	[2]
Total annual fund operating expenses after waiving fees and/or reimbursing expenses	rr_NetExpensesOverAssets	0.40%
1 year	rr_ExpenseExampleYear01	41
3 years	rr_ExpenseExampleYear03	132
5 years	rr_ExpenseExampleYear05	235
10 years	rr_ExpenseExampleYear10	537
Administrative | Western Asset Institutional Liquid Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
2002	rr_AnnualReturn2002	1.85%
2003	rr_AnnualReturn2003	1.10%
2004	rr_AnnualReturn2004	1.31%
2005	rr_AnnualReturn2005	3.20%
2006	rr_AnnualReturn2006	4.99%
2007	rr_AnnualReturn2007	5.32%
2008	rr_AnnualReturn2008	3.11%
2009	rr_AnnualReturn2009	0.68%
2010	rr_AnnualReturn2010	0.23%
2011	rr_AnnualReturn2011	0.17%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.15%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.34%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.03%
1 year	rr_AverageAnnualReturnYear01	0.17%
5 years	rr_AverageAnnualReturnYear05	1.88%
10 years	rr_AverageAnnualReturnYear10	2.18%

[1] "Other expenses" for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

[2] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.40% for Administrative Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

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Investor | Western Asset Institutional Cash Reserves

Western Asset Institutional Cash Reserves

Investment objective

The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Investor Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Investor Western Asset Institutional Cash Reserves Investor Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Investor Western Asset Institutional Cash Reserves Investor Shares
Management fees		0.20%
Distribution and service (12b-1) fees		0.10%
Other expenses	[1]	0.03%
Total annual fund operating expenses		0.33%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Investor Western Asset Institutional Cash Reserves Investor Shares	34	106	186	418

The fund invests in securities through an underlying fund: Prime Cash Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Prime Cash Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SA”).

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class O shares), a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.32

Worst quarter
(09/30/2011): 0.03

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.15

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	Since Inception	Inception date
Investor Western Asset Institutional Cash Reserves Institutional Shares	0.17%	1.83%	2.17%	Oct. 02, 2002

[1] "Other expenses" for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

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Administrative | Western Asset Institutional Government Reserves

Western Asset Institutional Government Reserves

Investment objective

The fund seeks maximum current income to the extent consistent with preservation of capital and the maintenance of liquidity.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Administrative Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Administrative Western Asset Institutional Government Reserves Administrative Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Administrative Western Asset Institutional Government Reserves Administrative Shares
Management fees		0.20%
Distribution and service (12b-1) fees		0.20%
Other expenses	[1]	0.02%
Total annual fund operating expenses		0.42%
Fees waived and/or expenses reimbursed	[2]	(0.02%)
Total annual fund operating expenses after waiving fees and/or reimbursing expenses		0.40%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Administrative Western Asset Institutional Government Reserves Administrative Shares	41	133	231	525

The fund invests in securities through an underlying fund: Government Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Government Portfolio.

Principal investment strategies

The fund is a money market fund which invests exclusively in short-term U.S. government obligations, including U.S. Treasuries and securities issued or guaranteed by the U.S. government or its agencies, authorities, instrumentalities or sponsored entities and in repurchase agreements. These securities may pay interest at fixed, floating or adjustable rates or may be issued at a discount. U.S. government obligations are not necessarily backed by the full faith and credit of the United States. Although the fund invests in U.S. government obligations, an investment in the fund is neither insured nor guaranteed by the U.S. government.

The fund invests in securities that, at the time of purchase, are rated by one or more rating agencies in the highest short term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information ("SAI").

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares (formerly Class A shares), a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.28

Worst quarter
(09/30/2011): 0.01

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.03

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Administrative Western Asset Institutional Government Reserves Institutional Shares	0.04%	1.58%	1.94%

[1] "Other expenses" for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

[2] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.40% for Administrative Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Liquid Reserves Shares | Western Asset Institutional Liquid Reserves

Western Asset Institutional Liquid Reserves

Investment objective

The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income as is consistent with preservation of capital.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold SVB Securities Liquid Reserves Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Liquid Reserves Shares Western Asset Institutional Liquid Reserves SVB Securities Liquid Reserves Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your
investment) (%)

Annual Fund Operating Expenses		Liquid Reserves Shares Western Asset Institutional Liquid Reserves SVB Securities Liquid Reserves Shares
Management fees		0.21%
Distribution and service (12b-1) fees		0.45%
Other expenses		0.04%
Total annual fund operating expenses		0.70%
Fees waived and/or expenses reimbursed	[1]	(0.10%)
Total annual fund operating expenses after waiving fees and/or reimbursing expenses		0.60%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Liquid Reserves Shares Western Asset Institutional Liquid Reserves SVB Securities Liquid Reserves Shares	61	203	369	850

The fund invests in securities through an underlying fund: Liquid Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of Liquid Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which invests in high quality, U.S. dollar-denominated short-term debt securities that, at the time of purchase, are rated by one or more rating agencies in the highest short-term rating category or, if not rated, are determined by the subadviser to be of equivalent quality.

The fund may invest in all types of money market instruments, including bank obligations, commercial paper and asset-backed securities, structured investments, repurchase agreements and other short-term debt securities. These instruments may be issued or guaranteed by all types of issuers, including U.S. and foreign banks and other private issuers, the U.S. government or any of its agencies or instrumentalities, U.S. states and municipalities, or foreign governments. These securities may pay interest at fixed, floating or adjustable rates, or may be issued at a discount. The fund may invest without limit in bank obligations, such as certificates of deposit, fixed time deposits and bankers’ acceptances. The fund generally limits its investments in foreign securities to U.S. dollar denominated obligations of issuers, including banks and foreign governments, located in the major industrialized countries, although with respect to bank obligations, the branches of the banks issuing the obligations may be located in The Bahamas or the Cayman Islands.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease, or if a fee limitation is changed or terminated.

Structured securities risk. The payment and credit qualities of structured securities derive from their underlying assets, and they may behave in ways not anticipated by the fund, or they may not receive tax, accounting or regulatory treatment anticipated by the fund.

Risks associated with concentration in the banking industry. The fund may invest a significant portion of its assets in obligations that are issued or backed by U.S. and non-U.S. banks, and thus will be more susceptible to negative events affecting the worldwide banking industry.

Foreign investments risk. The fund’s investments in securities of foreign issuers or issuers with significant exposure to foreign markets involve additional risk. Foreign countries in which the fund may invest may have markets that are less liquid, less regulated and more volatile than U.S. markets. The value of the fund’s investments may decline because of factors affecting the particular issuer as well as foreign markets and issuers generally, such as unfavorable government actions, and political or financial instability. Lack of information may also affect the value of these securities.

Prepayment or call risk. Many issuers have a right to prepay their securities. If interest rates fall, an issuer may exercise this right. If this happens, the fund will be forced to reinvest prepayment proceeds at a time when yields on securities available in the market are lower than the yield on the prepaid security.

Extension risk. If interest rates rise, repayments of fixed income securities may occur more slowly than anticipated by the market. This may drive the prices of these securities down because their interest rates are lower than the current interest rate and they remain outstanding longer.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for SVB Securities Liquid Reserves Shares. The table shows the average annual total returns of each class of the fund’s shares offered through this Prospectus that has been in operation for at least one full calendar year. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available by calling your Service Agent at 1-800-303-7371.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.22

Worst quarter
(03/31/2011): 0.00

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.01

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Liquid Reserves Shares Western Asset Institutional Liquid Reserves SVB Securities Liquid Reserves Shares	0.01%	1.56%	1.80%

[1] The manager has agreed to waive fees and/or reimburse operating expenses (other than interest, brokerage, taxes, extraordinary expenses and acquired fund fees and expenses) so that total annual operating expenses are not expected to exceed 0.60% for SVB Securities Liquid Reserves Shares. This arrangement cannot be terminated prior to December 31, 2014 without the Board of Trustees' consent. Additional amounts may be voluntarily waived and/or reimbursed from time to time. The manager is permitted to recapture any such amounts waived and/or reimbursed to the class during the same fiscal year if the class' total annual operating expenses have fallen to a level below the limit described above.

XML

Investor | Western Asset Institutional U.S. Treasury Reserves

Western Asset Institutional U.S. Treasury Reserves

Investment objective

The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income from U.S. government obligations as is consistent with preservation of capital.

Fees and expenses of the fund

The accompanying table describes the fees and expenses that you may pay if you buy and hold Investor Shares of the fund.

Shareholder fees (fees paid directly from your investment) (%)

Shareholder Fees	Investor Western Asset Institutional U.S. Treasury Reserves Investor Shares
Maximum sales charge (load) imposed on purchases	none
Maximum deferred sales charge (load)	none

Annual fund operating expenses (expenses that you pay each year as a percentage of the value of
your investment) (%)

Annual Fund Operating Expenses		Investor Western Asset Institutional U.S. Treasury Reserves Investor Shares
Management fees		0.18%
Distribution and service (12b-1) fees		0.10%
Other expenses	[1]	0.02%
Total annual fund operating expenses		0.30%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes:

• You invest $10,000 in the fund for the time periods indicated

• Your investment has a 5% return each year and the fund’s operating expenses remain the same

• You reinvest all distributions and dividends without a sales charge

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Number of years you own your shares ($)

Expense Example (USD $)	1 year	3 years	5 years	10 years
Investor Western Asset Institutional U.S. Treasury Reserves Investor Shares	31	97	169	380

The fund invests in securities through an underlying fund: U.S. Treasury Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of U.S. Treasury Reserves Portfolio.

Principal investment strategies

The fund is a money market fund which invests all of its assets in direct obligations of the U.S. Treasury, including U.S. Treasury bills, notes and bonds. The fund will not enter into repurchase agreements, but may enter into reverse repurchase agreements to satisfy redemption requests or for other temporary or emergency purposes. Although the fund invests in U.S. government obligations, an investment in the fund is neither insured nor guaranteed by the U.S. government.

As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.

Certain risks

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.

Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.

There is no assurance that the fund will meet its investment objective.

The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as:

Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time.

Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline.

Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield.

Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated.

Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect.

Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations.

These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).

Performance

The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Investor Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Investor Shares because there were no Investor Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002.

The fund’s past performance is not necessarily an indication of how the fund will perform in the future.

Total returns (%)

Calendar Years ended December 31

Best quarter
(12/31/2006): 1.22

Worst quarter
(03/31/2010): 0.00

The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.01

Average annual total returns (for periods ended December 31, 2011) (%)

Average Annual Total Returns	1 year	5 years	10 years
Investor Western Asset Institutional U.S. Treasury Reserves Institutional Shares	0.02%	1.22%	1.68%

[1] "Other expenses" for Investor Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	LEGG MASON PARTNERS INSTITUTIONAL TRUST
Prospectus Date	rr_ProspectusDate	Dec. 28, 2012
Administrative | Western Asset Institutional U.S. Treasury Reserves
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Western Asset Institutional U.S. Treasury Reserves
Objective [Heading]	rr_ObjectiveHeading	Investment objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund’s investment objective is to provide shareholders with liquidity and as high a level of current income from U.S. government obligations as is consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses of the fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The accompanying table describes the fees and expenses that you may pay if you buy and hold Administrative Shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment) (%)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual fund operating expenses (expenses that you pay each year as a percentage of the value of your investment) (%)
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fund invests in securities through an underlying fund: U.S. Treasury Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of U.S. Treasury Reserves Portfolio.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other expenses” for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes: You invest $10,000 in the fund for the time periods indicated Your investment has a 5% return each year and the fund’s operating expenses remain the same You reinvest all distributions and dividends without a sales charge Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Number of years you own your shares ($)
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	The fund invests in securities through an underlying fund: U.S. Treasury Reserves Portfolio. The information above reflects the direct expenses of the fund and its allocated share of expenses of U.S. Treasury Reserves Portfolio.
Strategy [Heading]	rr_StrategyHeading	Principal investment strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is a money market fund which invests all of its assets in direct obligations of the U.S. Treasury, including U.S. Treasury bills, notes and bonds. The fund will not enter into repurchase agreements, but may enter into reverse repurchase agreements to satisfy redemption requests or for other temporary or emergency purposes. Although the fund invests in U.S. government obligations, an investment in the fund is neither insured nor guaranteed by the U.S. government. As a money market fund, the fund tries to maintain a share price of $1.00, and must follow strict rules as to the credit quality, liquidity, diversification and maturity of its investments. Where required by these rules, the fund’s subadviser or Board of Trustees (the “Board”) will decide whether a security should be held or sold in the event of credit downgrades or certain other events occurring after purchase.
Risk [Heading]	rr_RiskHeading	Certain risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund. Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken. There is no assurance that the fund will meet its investment objective. The fund could underperform other short-term debt instruments or money market funds, or you could lose money, as a result of risks such as: Market and interest rate risk. There may be changes in interest rates, lack of liquidity or other disruptions in the bond markets or other adverse market events and conditions. The financial crisis that began in 2008 has caused a significant decline in the value and liquidity of many securities of issuers worldwide. In response to the crisis, the U.S. and other governments and the Federal Reserve and certain foreign central banks have taken steps to support financial markets. The withdrawal of this support, failure of efforts in response to the crisis, or investor perception that these efforts are not succeeding could negatively affect financial markets generally as well as the value and liquidity of certain securities. In addition, policy and legislative changes in the United States and in other countries are changing many aspects of financial regulation. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Credit risk. An issuer or obligor of a security held by the fund or a counterparty to a financial contract with the fund may default or its credit may be downgraded, or the value of assets underlying a security may decline. Yield risk. The amount of income received by the fund will go up or down depending on day-to-day variations in short-term interest rates, and when interest rates are very low the fund’s expenses could absorb all or a significant portion of the fund’s income. If interest rates increase, the fund’s yield may not increase proportionately. For example, the fund’s manager may discontinue any temporary voluntary fee limitation or recoup amounts previously waived and/or reimbursed. In addition, the recent adoption of more stringent regulations governing the management of money market funds could have a negative effect on the fund’s yield. Risk of increase in expenses. Your actual costs of investing in the fund may be higher than the expenses shown in “Annual fund operating expenses” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets are lower than estimated, or if a fee limitation is changed or terminated. Portfolio selection risk. The value of your investment may decrease if the subadviser’s judgment about the quality, relative yield or value of, or market trends affecting, a particular security or sector, or about interest rates generally, is incorrect. Redemption risk. The fund may experience heavy redemptions, particularly during periods of declining or illiquid markets, that could cause the fund to liquidate its assets at inopportune times or at a loss or depressed value and that could affect the fund’s ability to maintain a $1.00 share price. In addition, the fund may suspend redemptions when permitted by applicable regulations. These risks are discussed in more detail later in this Prospectus or in the statement of additional information (“SAI”).
Risk Lose Money [Text]	rr_RiskLoseMoney	Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Additionally, you should be aware that a very small number of money market funds in other fund complexes have, in the past, “broken the buck,” which means that investors did not receive $1.00 per share for their investment in those funds, and any money market fund may do so in the future. You should also be aware that the fund’s manager and its affiliates are under no obligation to provide financial support to the fund or take other measures to ensure that you receive $1.00 per share for your investment in the fund. You should not invest in the fund with the expectation that any such action will be taken.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ. The fund makes updated performance information available at the fund’s website, http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class), or by calling the fund at 1-877-721-1926 or 1-203-703-6002. The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Administrative Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-721-1926 or 1-203-703-6002
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.leggmason.com/individualinvestors/products/mutual-funds/annualized_performance (select share class)
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The fund’s past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Total returns (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Calendar Years ended December 31 Best quarter (12/31/2006): 1.22 Worst quarter (03/31/2010): 0.00 The year-to-date return as of the most recent calendar quarter, which ended 09/30/2012, was 0.01
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The bar chart shows changes in the fund’s performance from year to year for Institutional Shares, a class of the fund that is invested in the same portfolio of securities as Administrative Shares. The table shows the average annual total returns of Institutional Shares. No performance information is presented for Administrative Shares because there were no Administrative Shares outstanding during the calendar years shown. Performance for classes other than those shown may vary from the performance shown to the extent the expenses for those classes differ.
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns (for periods ended December 31, 2011) (%)
Administrative | Western Asset Institutional U.S. Treasury Reserves | Administrative Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.18%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.20%
Other expenses	rr_OtherExpensesOverAssets	0.02%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.40%
1 year	rr_ExpenseExampleYear01	41
3 years	rr_ExpenseExampleYear03	129
5 years	rr_ExpenseExampleYear05	225
10 years	rr_ExpenseExampleYear10	506
Administrative | Western Asset Institutional U.S. Treasury Reserves | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
2002	rr_AnnualReturn2002	1.52%
2003	rr_AnnualReturn2003	0.83%
2004	rr_AnnualReturn2004	1.02%
2005	rr_AnnualReturn2005	2.79%
2006	rr_AnnualReturn2006	4.59%
2007	rr_AnnualReturn2007	4.42%
2008	rr_AnnualReturn2008	1.58%
2009	rr_AnnualReturn2009	0.13%
2010	rr_AnnualReturn2010	0.02%
2011	rr_AnnualReturn2011	0.02%
Year to Date Return, Label	rr_YearToDateReturnLabel	The year-to-date return as of the most recent calendar quarter
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.01%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.22%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2010
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.00%
1 year	rr_AverageAnnualReturnYear01	0.02%
5 years	rr_AverageAnnualReturnYear05	1.22%
10 years	rr_AverageAnnualReturnYear10	1.68%

[1] "Other expenses" for Administrative Shares are estimated for the current fiscal year. Actual expenses may differ from estimates.