485BPOS

As filed with the Securities and Exchange Commission on November 28, 2018

Securities Act of 1933 Registration No. 2-80543

Investment Company Act of 1940 Registration No. 811-03605

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933	☒
Pre-Effective Amendment No.	☐
Post-Effective Amendment No. 94	☒

and/or

REGISTRATION STATEMENT

UNDER

THE INVESTMENT COMPANY ACT OF 1940

Amendment No. 116 ☒

(Check appropriate box or boxes)

NORTHERN INSTITUTIONAL FUNDS

(Exact Name of Registrant as Specified in Charter)

50 South LaSalle Street

Chicago, Illinois 60603

(Address of Principal Executive Offices)

800-637-1380

(Registrant’s Telephone Number, including Area Code)

Name and Address of Agent for Service:   Diana E. McCarthy Drinker Biddle & Reath LLP One Logan Square, Suite 2000 Philadelphia, Pennsylvania 19103-6996 with a copy to:   Kevin P. O’Rourke Jose J. Del Real, Esq. The Northern Trust Company 50 South LaSalle Street Chicago, Illinois 60603

It Is Proposed That This Filing Become Effective (Check Appropriate Box):

☒ immediately upon filing pursuant to paragraph (b)

☐ On [date] pursuant to paragraph (b)

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

☐ On (date) pursuant to paragraph (a)(1)

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

☐ On (date) pursuant to paragraph (a)(2)

If appropriate, check the following box:

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

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 certifies that it meets all of the requirements for effectiveness of this Post-Effective Amendment No. 94 to its Registration Statement under Rule 485(b) under the Securities Act and has duly caused this Post-Effective Amendment No. 94 to its Registration Statement to be signed on its behalf by the undersigned, duly authorized, in the City of Chicago and State of Illinois on the 28^th day of November, 2018.

NORTHERN INSTITUTIONAL FUNDS
By:	/s/ Peter K. Ewing
	Peter K. Ewing
	President

Pursuant to the requirements of the Securities Act, this Post-Effective Amendment No. 94 to Registrant’s Registration Statement has been signed below by the following persons in the capacities and on the date indicated.

Name	Title	Date
/s/ Peter K. Ewing	President (Principal Executive Officer)	November 28, 2018
Peter K. Ewing
/s/ Randal E. Rein	Treasurer (Principal Financial Officer and	November 28, 2018
Randal E. Rein	Principal Accounting Officer)
* William L. Bax	Trustee	November 28, 2018
William L. Bax
* Mark G. Doll	Trustee	November 28, 2018
Mark G. Doll
* Sandra Polk Guthman	Trustee	November 28, 2018
Sandra Polk Guthman
* Thomas A. Kloet	Trustee	November 28, 2018
Thomas A. Kloet
* David R. Martin	Trustee	November 28, 2018
David R. Martin
* Cynthia R. Plouché	Trustee	November 28, 2018
Cynthia R. Plouché
* Mary Jacobs Skinner	Trustee	November 28, 2018
Mary Jacobs Skinner
* Casey J. Sylla	Trustee	November 28, 2018
Casey J. Sylla
* Shundrawn A. Thomas	Trustee	November 28, 2018
Shundrawn A. Thomas
/s/ Peter K. Ewing
* By Peter K. Ewing    Attorney-In-Fact

Exhibit Index

Exhibit No.	Description
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

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	May 31, 2018
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Central Index Key	dei_EntityCentralIndexKey	0000710124
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Sep. 14, 2018
Document Effective Date	dei_DocumentEffectiveDate	Nov. 13, 2018
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Entity Inv Company Type	dei_EntityInvCompanyType	N-1A

XML

Shares | NORTHERN INSTITUTIONAL TREASURY PORTFOLIO

<b>TREASURY PORTFOLIO—SHARES </b>

<b>INVESTMENT OBJECTIVE </b>

The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing its net assets, under normal market conditions, exclusively in U.S. Treasury securities and related repurchase agreements and other securities that limit their investments to, or are backed by, U.S. Treasury securities.

<b>FEES AND EXPENSES OF THE PORTFOLIO </b>

This table describes the fees and expenses that you may pay if you buy and hold Shares of the Portfolio.

<b>Shareholder Fees </b>(fees paid directly from your investment)

Shareholder Fees	Shares NORTHERN INSTITUTIONAL TREASURY PORTFOLIO Shares USD ($)
Shareholder Fees (fees paid directly from your investment)	none

<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)

Annual Fund Operating Expenses	Shares NORTHERN INSTITUTIONAL TREASURY PORTFOLIO Shares
Management Fees	0.13%
Other Expenses	0.03%
Transfer Agent Fees	0.02%
Service Fees	none
Other Operating Expenses	0.01%
Total Annual Portfolio Operating Expenses	0.16%
Expense Reimbursement	(0.01%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	0.15%

<b>EXAMPLE </b>

The following Example is intended to help you compare the cost of investing in Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example	1 Year	3 Years	5 Years	10 Years
Shares | NORTHERN INSTITUTIONAL TREASURY PORTFOLIO | Shares | USD ($)	15	51	89	204

<b>PRINCIPAL INVESTMENT STRATEGIES</b>

The Portfolio seeks to achieve its objective by investing, under normal circumstances, its total assets exclusively in:

• Cash;

• Short-term bills, notes, including floating rate notes, and other obligations issued or guaranteed by the U.S. Treasury (“Treasury Obligations”); and

• Repurchase agreements collateralized fully by cash or Treasury Obligations.

The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in Treasury Obligations and repurchase agreements collateralized solely by Treasury Obligations.

The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur.

The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.

<b>PRINCIPAL RISKS</b>

CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect.

CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders.

DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities.

INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities.

INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio.

LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would.

MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results.

MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets.

PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities.

STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV.

VARIABLE OR FLOATING RATE INSTRUMENTS RISK is the risk that securities with variable or floating rates can be less sensitive to interest rate changes than securities with fixed interest rates, but may decline in value and negatively impact the Portfolio, particularly if changes in prevailing interest rates are more frequent or sudden than the rate changes for the variable or floating rate securities, which only occur periodically. Although variable and floating rate securities are less sensitive to interest rate risk than fixed-rate securities, they are subject to credit risk and default risk, which could impede their value.

You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.

<b>PORTFOLIO PERFORMANCE </b>

The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Shares from year to year.

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

Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.

<b>CALENDAR YEAR TOTAL RETURN (SHARES)* </b>

* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.25% in the fourth quarter of 2017, and the lowest quarterly return was 0.00% in the second quarter of 2015.

<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)

Average Annual Total Returns	Inception Date	1-Year	5-Year	Since Inception
Shares | NORTHERN INSTITUTIONAL TREASURY PORTFOLIO | Shares	Nov. 05, 2008	0.77%	0.21%	0.14%

The 7-day yield for Shares of the Portfolio as of December 31, 2017: 1.17%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.

[1] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.15%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Shares | NORTHERN INSTITUTIONAL TREASURY PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>TREASURY PORTFOLIO—SHARES </b>
Objective [Heading]	rr_ObjectiveHeading	<b>INVESTMENT OBJECTIVE </b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing its net assets, under normal market conditions, exclusively in U.S. Treasury securities and related repurchase agreements and other securities that limit their investments to, or are backed by, U.S. Treasury securities.
Expense [Heading]	rr_ExpenseHeading	<b>FEES AND EXPENSES OF THE PORTFOLIO </b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Shares of the Portfolio.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<b>Shareholder Fees </b>(fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>EXAMPLE </b>
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is intended to help you compare the cost of investing in Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	<b>PRINCIPAL INVESTMENT STRATEGIES</b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Portfolio seeks to achieve its objective by investing, under normal circumstances, its total assets exclusively in: Cash; Short-term bills, notes, including floating rate notes, and other obligations issued or guaranteed by the U.S. Treasury (“Treasury Obligations”); and Repurchase agreements collateralized fully by cash or Treasury Obligations. The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in Treasury Obligations and repurchase agreements collateralized solely by Treasury Obligations. The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur. The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.
Risk [Heading]	rr_RiskHeading	<b>PRINCIPAL RISKS</b>
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect. CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders. DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities. INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities. INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio. LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results. MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets. PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities. STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV. VARIABLE OR FLOATING RATE INSTRUMENTS RISK is the risk that securities with variable or floating rates can be less sensitive to interest rate changes than securities with fixed interest rates, but may decline in value and negatively impact the Portfolio, particularly if changes in prevailing interest rates are more frequent or sudden than the rate changes for the variable or floating rate securities, which only occur periodically. Although variable and floating rate securities are less sensitive to interest rate risk than fixed-rate securities, they are subject to credit risk and default risk, which could impede their value. You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.
Risk Lose Money [Text]	rr_RiskLoseMoney	<b>You could lose money by investing in the Portfolio.</b>
Risk Money Market Fund May Not Preserve Dollar [Text]	rr_RiskMoneyMarketFundMayNotPreserveDollar	<b>Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so.</b>
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	<b>An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank.</b>
Risk Money Market Fund Sponsor May Not Provide Support [Text]	rr_RiskMoneyMarketFundSponsorMayNotProvideSupport	<b>The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.</b>
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>PORTFOLIO PERFORMANCE </b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Shares from year to year. The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future. Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Shares from year to year.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-637-1380
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	northerntrust.com/institutional
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	<b>CALENDAR YEAR TOTAL RETURN (SHARES)* </b>
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.25% in the fourth quarter of 2017, and the lowest quarterly return was 0.00% in the second quarter of 2015.
Performance Table Heading	rr_PerformanceTableHeading	<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	The 7-day yield for Shares of the Portfolio as of December 31, 2017: 1.17%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.
Money Market Seven Day Yield, Caption [Text]	rr_MoneyMarketSevenDayYieldCaption	The 7-day yield for Shares of the Portfolio as of December 31, 2017
Money Market Seven Day Yield Phone	rr_MoneyMarketSevenDayYieldPhone	800-637-1380
Shares | NORTHERN INSTITUTIONAL TREASURY PORTFOLIO | Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.13%
Other Expenses	rr_OtherExpensesOverAssets	0.03%
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	none
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.01%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.16%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.01%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.15%
1 Year	rr_ExpenseExampleYear01	$ 15
3 Years	rr_ExpenseExampleYear03	51
5 Years	rr_ExpenseExampleYear05	89
10 Years	rr_ExpenseExampleYear10	$ 204
2009	rr_AnnualReturn2009	0.11%
2010	rr_AnnualReturn2010	0.07%
2011	rr_AnnualReturn2011	0.02%
2012	rr_AnnualReturn2012	0.02%
2013	rr_AnnualReturn2013	0.01%
2014	rr_AnnualReturn2014	0.01%
2015	rr_AnnualReturn2015	0.02%
2016	rr_AnnualReturn2016	0.25%
2017	rr_AnnualReturn2017	0.77%
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to date total return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest quarterly return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2017
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.25%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest quarterly return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Jun. 30, 2015
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	none
1-Year	rr_AverageAnnualReturnYear01	0.77%
5-Year	rr_AverageAnnualReturnYear05	0.21%
Since Inception	rr_AverageAnnualReturnSinceInception	0.14%
Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 05, 2008
Money Market Seven Day Yield	rr_MoneyMarketSevenDayYield	1.17%

[1] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.15%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

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Service and Premier Shares | NORTHERN INSTITUTIONAL TREASURY PORTFOLIO

<b>TREASURY PORTFOLIO—SERVICE SHARES AND PREMIER SHARES </b>

<b>INVESTMENT OBJECTIVE </b>

The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing its net assets, under normal market conditions, exclusively in U.S. Treasury securities and related repurchase agreements and other securities that limit their investments to, or are backed by, U.S. Treasury securities.

<b>FEES AND EXPENSES OF THE PORTFOLIO </b>

This table describes the fees and expenses that you may pay if you buy and hold Service Shares and Premier Shares of the Portfolio.

<b>Shareholder Fees </b>(fees paid directly from your investment)

Shareholder Fees - Service and Premier Shares - NORTHERN INSTITUTIONAL TREASURY PORTFOLIO - USD ($)	Service Shares	Premier Shares
Shareholder Fees (fees paid directly from your investment)	none	none

<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)

Annual Fund Operating Expenses - Service and Premier Shares - NORTHERN INSTITUTIONAL TREASURY PORTFOLIO		Service Shares	Premier Shares
Management Fees		0.13%	0.13%
Other Expenses		0.28%	0.08%
Transfer Agent Fees		0.02%	0.02%
Service Fees		0.25%	0.05%
Other Operating Expenses		0.01%	0.01%
Total Annual Portfolio Operating Expenses		0.41%	0.21%
Expense Reimbursement	[1]	(0.01%)	(0.01%)
Total Annual Portfolio Operating Expenses After Expense Reimbursement		0.40%	0.20%

<b>EXAMPLE </b>

The following Example is intended to help you compare the cost of investing in Service Shares and Premier Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example - Service and Premier Shares - NORTHERN INSTITUTIONAL TREASURY PORTFOLIO - USD ($)	1 Year	3 Years	5 Years	10 Years
Service Shares	41	131	229	517
Premier Shares	20	67	117	267

<b>PRINCIPAL INVESTMENT STRATEGIES </b>

The Portfolio seeks to achieve its objective by investing, under normal circumstances, its total assets exclusively in:

• Cash;

• Short-term bills, notes, including floating rate notes, and other obligations issued or guaranteed by the U.S. Treasury (“Treasury Obligations”); and

• Repurchase agreements collateralized fully by cash or Treasury Obligations.

The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in Treasury Obligations and repurchase agreements collateralized solely by Treasury Obligations.

The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur.

The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.

<b>PRINCIPAL RISKS </b>

CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect.

CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders.

DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities.

INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities.

INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio.

LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would.

MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results.

MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets.

PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities.

STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV.

VARIABLE OR FLOATING RATE INSTRUMENTS RISK is the risk that securities with variable or floating rates can be less sensitive to interest rate changes than securities with fixed interest rates, but may decline in value and negatively impact the Portfolio, particularly if changes in prevailing interest rates are more frequent or sudden than the rate changes for the variable or floating rate securities, which only occur periodically. Although variable and floating rate securities are less sensitive to interest rate risk than fixed-rate securities, they are subject to credit risk and default risk, which could impede their value.

You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.

<b>PORTFOLIO PERFORMANCE </b>

The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing the performance of the Portfolio’s Premier Shares for the calendar year ended December 31, 2017. As of December 31, 2017 there were no Service Shares outstanding for the Portfolio.

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

Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.

<b>CALENDAR YEAR TOTAL RETURN (PREMIER SHARES)* </b>

* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.24% in the fourth quarter of 2017, and the lowest quarterly return was 0.10% in the first quarter of 2017.

<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)

Average Annual Total Returns	Inception Date	1-Year	Since Inception
Service and Premier Shares | NORTHERN INSTITUTIONAL TREASURY PORTFOLIO | Premier Shares	Aug. 01, 2016	0.72%	0.57%

The 7-day yield for Premier Shares of the Portfolio as of December 31,2017: 1.12%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.

[1] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.15%. The “Total Annual Portfolio Operating Expenses After Expense Reimbursement” may be higher than the contractual limitation as a result of certain Portfolio expenses, including but not limited to service fees, that are not reimbursed. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Service and Premier Shares | NORTHERN INSTITUTIONAL TREASURY PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>TREASURY PORTFOLIO—SERVICE SHARES AND PREMIER SHARES </b>
Objective [Heading]	rr_ObjectiveHeading	<b>INVESTMENT OBJECTIVE </b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing its net assets, under normal market conditions, exclusively in U.S. Treasury securities and related repurchase agreements and other securities that limit their investments to, or are backed by, U.S. Treasury securities.
Expense [Heading]	rr_ExpenseHeading	<b>FEES AND EXPENSES OF THE PORTFOLIO </b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Service Shares and Premier Shares of the Portfolio.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<b>Shareholder Fees </b>(fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>EXAMPLE </b>
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is intended to help you compare the cost of investing in Service Shares and Premier Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	<b>PRINCIPAL INVESTMENT STRATEGIES </b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Portfolio seeks to achieve its objective by investing, under normal circumstances, its total assets exclusively in: Cash; Short-term bills, notes, including floating rate notes, and other obligations issued or guaranteed by the U.S. Treasury (“Treasury Obligations”); and Repurchase agreements collateralized fully by cash or Treasury Obligations. The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in Treasury Obligations and repurchase agreements collateralized solely by Treasury Obligations. The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur. The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.
Risk [Heading]	rr_RiskHeading	<b>PRINCIPAL RISKS </b>
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect. CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders. DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities. INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities. INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio. LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results. MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets. PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities. STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV. VARIABLE OR FLOATING RATE INSTRUMENTS RISK is the risk that securities with variable or floating rates can be less sensitive to interest rate changes than securities with fixed interest rates, but may decline in value and negatively impact the Portfolio, particularly if changes in prevailing interest rates are more frequent or sudden than the rate changes for the variable or floating rate securities, which only occur periodically. Although variable and floating rate securities are less sensitive to interest rate risk than fixed-rate securities, they are subject to credit risk and default risk, which could impede their value. You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.
Risk Lose Money [Text]	rr_RiskLoseMoney	<b>You could lose money by investing in the Portfolio.</b>
Risk Money Market Fund May Not Preserve Dollar [Text]	rr_RiskMoneyMarketFundMayNotPreserveDollar	<b>Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. </b>
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	<b>An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank.</b>
Risk Money Market Fund Sponsor May Not Provide Support [Text]	rr_RiskMoneyMarketFundSponsorMayNotProvideSupport	<b>The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.</b>
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>PORTFOLIO PERFORMANCE </b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing the performance of the Portfolio’s Premier Shares for the calendar year ended December 31, 2017. As of December 31, 2017 there were no Service Shares outstanding for the Portfolio. The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future. Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing the performance of the Portfolio’s Premier Shares for the calendar year ended December 31, 2017.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-637-1380
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	northerntrust.com/institutional
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	<b>CALENDAR YEAR TOTAL RETURN (PREMIER SHARES)* </b>
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.24% in the fourth quarter of 2017, and the lowest quarterly return was 0.10% in the first quarter of 2017.
Performance Table Heading	rr_PerformanceTableHeading	<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	The 7-day yield for Premier Shares of the Portfolio as of December 31,2017: 1.12%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.
Money Market Seven Day Yield, Caption [Text]	rr_MoneyMarketSevenDayYieldCaption	The 7-day yield for Premier Shares of the Portfolio as of December 31,2017
Money Market Seven Day Yield Phone	rr_MoneyMarketSevenDayYieldPhone	800-637-1380
Service and Premier Shares | NORTHERN INSTITUTIONAL TREASURY PORTFOLIO | Service Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.13%
Other Expenses	rr_OtherExpensesOverAssets	0.28%
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	0.25%
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.01%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.41%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.01%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.40%
1 Year	rr_ExpenseExampleYear01	$ 41
3 Years	rr_ExpenseExampleYear03	131
5 Years	rr_ExpenseExampleYear05	229
10 Years	rr_ExpenseExampleYear10	517
Service and Premier Shares | NORTHERN INSTITUTIONAL TREASURY PORTFOLIO | Premier Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.13%
Other Expenses	rr_OtherExpensesOverAssets	0.08%
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	0.05%
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.01%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.21%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.01%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.20%
1 Year	rr_ExpenseExampleYear01	$ 20
3 Years	rr_ExpenseExampleYear03	67
5 Years	rr_ExpenseExampleYear05	117
10 Years	rr_ExpenseExampleYear10	$ 267
2017	rr_AnnualReturn2017	0.72%
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to date total return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest quarterly return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2017
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.24%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest quarterly return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2017
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.10%
1-Year	rr_AverageAnnualReturnYear01	0.72%
Since Inception	rr_AverageAnnualReturnSinceInception	0.57%
Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 01, 2016
Money Market Seven Day Yield	rr_MoneyMarketSevenDayYield	1.12%

[1] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.15%. The “Total Annual Portfolio Operating Expenses After Expense Reimbursement” may be higher than the contractual limitation as a result of certain Portfolio expenses, including but not limited to service fees, that are not reimbursed. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Shares | Treasury Instruments Portfolio

<b>TREASURY INSTRUMENTS PORTFOLIO—SHARES </b>

<b>INVESTMENT OBJECTIVE </b>

The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing its net assets, under normal conditions, exclusively in U.S. Treasury securities.

<b>FEES AND EXPENSES OF THE PORTFOLIO </b>

This table describes the fees and expenses that you may pay if you buy and hold Shares of the Portfolio.

<b>Shareholder Fees </b>(fees paid directly from your investment)

Shareholder Fees	Shares Treasury Instruments Portfolio Shares USD ($)
Shareholder Fees (fees paid directly from your investment)	none

<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)

Annual Fund Operating Expenses	Shares Treasury Instruments Portfolio Shares
Management Fees	0.18%
Other Expenses	0.21%	[1]
Transfer Agent Fees	0.02%
Service Fees	none
Other Operating Expenses	0.19%
Total Annual Portfolio Operating Expenses	0.39%
Expense Reimbursement	(0.19%)	[2]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	0.20%

<b>EXAMPLE </b>

The following Example is intended to help you compare the cost of investing in Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example	1 Year	3 Years
Shares | Treasury Instruments Portfolio | Shares | USD ($)	20	106

<b>PRINCIPAL INVESTMENT STRATEGIES </b>

The Portfolio seeks to achieve its objective by investing, under normal circumstances, its total assets exclusively in:

• Cash; and

• Short-term bills, notes and other obligations issued or guaranteed by the U.S. Treasury (“Treasury Obligations”).

The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in Treasury Obligations.

The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur.

The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit and interest rate risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.

<b>PRINCIPAL RISKS </b>

CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect.

CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders.

INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities.

INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio.

LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would.

MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results.

MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets.

STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV.

You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.

<b>PORTFOLIO PERFORMANCE</b>

The Portfolio is new and does not yet have a full calendar year of performance. After the Portfolio has been in operation for a full calendar year, total return information will be presented. Updated performance information, which is accessible on the Portfolio’s web site at northerntrust.com/institutional or by calling 800-637-1380, will provide some indication of the risks of investing in the Portfolio.

[1] “Other Expenses” are based on estimated amounts for the current fiscal year.

[2] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.20%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Shares | Treasury Instruments Portfolio
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>TREASURY INSTRUMENTS PORTFOLIO—SHARES </b>
Objective [Heading]	rr_ObjectiveHeading	<b>INVESTMENT OBJECTIVE </b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing its net assets, under normal conditions, exclusively in U.S. Treasury securities.
Expense [Heading]	rr_ExpenseHeading	<b>FEES AND EXPENSES OF THE PORTFOLIO </b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Shares of the Portfolio.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<b>Shareholder Fees </b>(fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other Expenses” are based on estimated amounts for the current fiscal year.
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>EXAMPLE </b>
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is intended to help you compare the cost of investing in Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	<b>PRINCIPAL INVESTMENT STRATEGIES </b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Portfolio seeks to achieve its objective by investing, under normal circumstances, its total assets exclusively in: Cash; and Short-term bills, notes and other obligations issued or guaranteed by the U.S. Treasury (“Treasury Obligations”). The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in Treasury Obligations. The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur. The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit and interest rate risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.
Risk [Heading]	rr_RiskHeading	<b>PRINCIPAL RISKS </b>
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect. CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders. INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities. INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio. LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results. MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets. STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV. You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.
Risk Lose Money [Text]	rr_RiskLoseMoney	<b>You could lose money by investing in the Portfolio.</b>
Risk Money Market Fund May Not Preserve Dollar [Text]	rr_RiskMoneyMarketFundMayNotPreserveDollar	<b>Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so.</b>
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	<b>An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank.</b>
Risk Money Market Fund Sponsor May Not Provide Support [Text]	rr_RiskMoneyMarketFundSponsorMayNotProvideSupport	<b>The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time. </b>
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>PORTFOLIO PERFORMANCE</b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The Portfolio is new and does not yet have a full calendar year of performance. After the Portfolio has been in operation for a full calendar year, total return information will be presented. Updated performance information, which is accessible on the Portfolio’s web site at northerntrust.com/institutional or by calling 800-637-1380, will provide some indication of the risks of investing in the Portfolio.
Performance One Year or Less [Text]	rr_PerformanceOneYearOrLess	The Portfolio is new and does not yet have a full calendar year of performance.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-637-1380
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	northerntrust.com/institutional
Shares | Treasury Instruments Portfolio | Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.18%
Other Expenses	rr_OtherExpensesOverAssets	0.21%	[1]
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	none
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.19%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.39%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.19%)	[2]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.20%
1 Year	rr_ExpenseExampleYear01	$ 20
3 Years	rr_ExpenseExampleYear03	$ 106

[1] “Other Expenses” are based on estimated amounts for the current fiscal year.

[2] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.20%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Service and Premier Shares | Treasury Instruments Portfolio

<b>TREASURY INSTRUMENTS PORTFOLIO—SERVICE SHARES AND PREMIER SHARES </b>

<b>INVESTMENT OBJECTIVE </b>

The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing its net assets, under normal conditions, exclusively in U.S. Treasury securities.

<b>FEES AND EXPENSES OF THE PORTFOLIO </b>

This table describes the fees and expenses that you may pay if you buy and hold Service Shares and Premier Shares of the Portfolio.

<b>Shareholder Fees </b>(fees paid directly from your investment)

Shareholder Fees - Service and Premier Shares - Treasury Instruments Portfolio - USD ($)	Service Shares	Premier Shares
Shareholder Fees (fees paid directly from your investment)	none	none

<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)

Annual Fund Operating Expenses - Service and Premier Shares - Treasury Instruments Portfolio		Service Shares	Premier Shares
Management Fees		0.18%	0.18%
Other Expenses	[1]	0.46%	0.71%
Transfer Agent Fees		0.02%	0.02%
Service Fees		0.25%	0.50%
Other Operating Expenses		0.19%	0.19%
Total Annual Portfolio Operating Expenses		0.64%	0.89%
Expense Reimbursement	[2]	(0.19%)	(0.19%)
Total Annual Portfolio Operating Expenses After Expense Reimbursement		0.45%	0.70%

<b>EXAMPLE </b>

The following Example is intended to help you compare the cost of investing in Service Shares and Premier Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example - Service and Premier Shares - Treasury Instruments Portfolio - USD ($)	1 Year	3 Years
Service Shares	46	186
Premier Shares	72	265

<b>PRINCIPAL INVESTMENT STRATEGIES </b>

The Portfolio seeks to achieve its objective by investing, under normal circumstances, its total assets exclusively in:

• Cash; and

• Short-term bills, notes and other obligations issued or guaranteed by the U.S. Treasury (“Treasury Obligations”).

The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in Treasury Obligations.

The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur.

The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit and interest rate risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.

<b>PRINCIPAL RISKS </b>

CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect.

CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders.

INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities.

INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio.

LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would.

MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results.

MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets.

STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV.

You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.

<b>PORTFOLIO PERFORMANCE </b>

The Portfolio is new and does not yet have a full calendar year of performance. After the Portfolio has been in operation for a full calendar year, total return information will be presented. Updated performance information, which is accessible on the Portfolio’s web site at northerntrust.com/institutional or by calling 800-637-1380, will provide some indication of the risks of investing in the Portfolio.

[1] “Other Expenses” are based on estimated amounts for the current fiscal year.

[2] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.20%. The “Total Annual Portfolio Operating Expenses After Expense Reimbursement” may be higher than the contractual limitation as a result of certain Portfolio expenses, including but not limited to service fees, that are not reimbursed. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Service and Premier Shares | Treasury Instruments Portfolio
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>TREASURY INSTRUMENTS PORTFOLIO—SERVICE SHARES AND PREMIER SHARES </b>
Objective [Heading]	rr_ObjectiveHeading	<b>INVESTMENT OBJECTIVE </b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing its net assets, under normal conditions, exclusively in U.S. Treasury securities.
Expense [Heading]	rr_ExpenseHeading	<b>FEES AND EXPENSES OF THE PORTFOLIO </b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Service Shares and Premier Shares of the Portfolio.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<b>Shareholder Fees </b>(fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other Expenses” are based on estimated amounts for the current fiscal year.
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>EXAMPLE </b>
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is intended to help you compare the cost of investing in Service Shares and Premier Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	<b>PRINCIPAL INVESTMENT STRATEGIES </b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Portfolio seeks to achieve its objective by investing, under normal circumstances, its total assets exclusively in: Cash; and Short-term bills, notes and other obligations issued or guaranteed by the U.S. Treasury (“Treasury Obligations”). The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in Treasury Obligations. The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur. The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit and interest rate risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.
Risk [Heading]	rr_RiskHeading	<b>PRINCIPAL RISKS </b>
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect. CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders. INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities. INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio. LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results. MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets. STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV. You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.
Risk Lose Money [Text]	rr_RiskLoseMoney	<b>You could lose money by investing in the Portfolio.</b>
Risk Money Market Fund May Not Preserve Dollar [Text]	rr_RiskMoneyMarketFundMayNotPreserveDollar	<b>Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. </b>
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	<b>An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank.</b>
Risk Money Market Fund Sponsor May Not Provide Support [Text]	rr_RiskMoneyMarketFundSponsorMayNotProvideSupport	<b>The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.</b>
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>PORTFOLIO PERFORMANCE </b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The Portfolio is new and does not yet have a full calendar year of performance. After the Portfolio has been in operation for a full calendar year, total return information will be presented. Updated performance information, which is accessible on the Portfolio’s web site at northerntrust.com/institutional or by calling 800-637-1380, will provide some indication of the risks of investing in the Portfolio.
Performance One Year or Less [Text]	rr_PerformanceOneYearOrLess	The Portfolio is new and does not yet have a full calendar year of performance.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-637-1380
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	northerntrust.com/institutional
Service and Premier Shares | Treasury Instruments Portfolio | Service Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.18%
Other Expenses	rr_OtherExpensesOverAssets	0.46%	[1]
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	0.25%
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.19%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.64%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.19%)	[2]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.45%
1 Year	rr_ExpenseExampleYear01	$ 46
3 Years	rr_ExpenseExampleYear03	186
Service and Premier Shares | Treasury Instruments Portfolio | Premier Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.18%
Other Expenses	rr_OtherExpensesOverAssets	0.71%	[1]
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	0.50%
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.19%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.89%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.19%)	[2]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.70%
1 Year	rr_ExpenseExampleYear01	$ 72
3 Years	rr_ExpenseExampleYear03	$ 265

[1] “Other Expenses” are based on estimated amounts for the current fiscal year.

[2] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.20%. The “Total Annual Portfolio Operating Expenses After Expense Reimbursement” may be higher than the contractual limitation as a result of certain Portfolio expenses, including but not limited to service fees, that are not reimbursed. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT PORTFOLIO

<b>U.S. GOVERNMENT PORTFOLIO—SHARES </b>

<b>INVESTMENT OBJECTIVE </b>

The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.

<b>FEES AND EXPENSES OF THE PORTFOLIO </b>

This table describes the fees and expenses that you may pay if you buy and hold Shares of the Portfolio.

<b>Shareholder Fees </b>(fees paid directly from your investment)

Shareholder Fees	Shares NORTHERN INSTITUTIONAL U.S. GOVERNMENT PORTFOLIO Shares USD ($)
Shareholder Fees (fees paid directly from your investment)	none

<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)

Annual Fund Operating Expenses	Shares NORTHERN INSTITUTIONAL U.S. GOVERNMENT PORTFOLIO Shares	[1]
Management Fees	0.23%
Other Expenses	0.04%
Transfer Agent Fees	0.02%
Service Fees	none
Other Operating Expenses	0.02%
Total Annual Portfolio Operating Expenses	0.27%
Expense Reimbursement	(0.02%)	[2]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	0.25%

<b>EXAMPLE </b>

The following Example is intended to help you compare the cost of investing in Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example	1 Year	3 Years	5 Years	10 Years
Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT PORTFOLIO | Shares | USD ($)	26	85	150	341

<b>PRINCIPAL INVESTMENT STRATEGIES </b>

The Portfolio seeks to achieve its objective by investing, under normal circumstances, substantially all (and at least 99.5%) of its total assets in cash, securities issued or guaranteed as to principal and interest by the U.S. government or by a person controlled or supervised by and acting as an instrumentality of the U.S. government pursuant to authority granted by the Congress of the United States or any certificate of deposit of any of the foregoing, and repurchase agreements that are fully collateralized by cash or such securities. The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in U.S. government securities and repurchase agreements collateralized solely by U.S. government securities.

The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur.

The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.

<b>PRINCIPAL RISKS </b>

CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect.

CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders.

DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities.

INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities.

INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio.

LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would.

MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results.

MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets.

PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities.

STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure) the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV.

U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future.

You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.

<b>PORTFOLIO PERFORMANCE </b>

The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Shares from year to year.

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

Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.

<b>CALENDAR YEAR TOTAL RETURN (SHARES)* </b>

* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.80% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the third quarter of 2015.

<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)

Average Annual Total Returns	Inception Date	1-Year	5-Year	10-Year	Since Inception
Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT PORTFOLIO | Shares	Oct. 29, 1985	0.68%	0.17%	0.29%	3.40%

The 7-day yield for Shares of the Portfolio as of December 31, 2017: 1.09%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.

[1] The expense information has been restated to reflect current fees.

[2] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.25%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>U.S. GOVERNMENT PORTFOLIO—SHARES </b>
Objective [Heading]	rr_ObjectiveHeading	<b>INVESTMENT OBJECTIVE </b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.
Expense [Heading]	rr_ExpenseHeading	<b>FEES AND EXPENSES OF THE PORTFOLIO </b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Shares of the Portfolio.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<b>Shareholder Fees </b>(fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	The expense information has been restated to reflect current fees.
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>EXAMPLE </b>
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is intended to help you compare the cost of investing in Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	<b>PRINCIPAL INVESTMENT STRATEGIES </b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Portfolio seeks to achieve its objective by investing, under normal circumstances, substantially all (and at least 99.5%) of its total assets in cash, securities issued or guaranteed as to principal and interest by the U.S. government or by a person controlled or supervised by and acting as an instrumentality of the U.S. government pursuant to authority granted by the Congress of the United States or any certificate of deposit of any of the foregoing, and repurchase agreements that are fully collateralized by cash or such securities. The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in U.S. government securities and repurchase agreements collateralized solely by U.S. government securities. The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur. The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.
Risk [Heading]	rr_RiskHeading	<b>PRINCIPAL RISKS </b>
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect. CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders. DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities. INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities. INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio. LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results. MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets. PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities. STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure) the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV. U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future. You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.
Risk Lose Money [Text]	rr_RiskLoseMoney	<b>You could lose money by investing in the Portfolio.</b>
Risk Money Market Fund May Not Preserve Dollar [Text]	rr_RiskMoneyMarketFundMayNotPreserveDollar	<b>Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so.</b>
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	<b>An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank.</b>
Risk Money Market Fund Sponsor May Not Provide Support [Text]	rr_RiskMoneyMarketFundSponsorMayNotProvideSupport	<b>The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.</b>
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>PORTFOLIO PERFORMANCE </b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Shares from year to year. The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future. Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Shares from year to year.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-637-1380
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	northerntrust.com/institutional
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	<b>CALENDAR YEAR TOTAL RETURN (SHARES)* </b>
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.80% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the third quarter of 2015.
Performance Table Heading	rr_PerformanceTableHeading	<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	The 7-day yield for Shares of the Portfolio as of December 31, 2017: 1.09%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.
Money Market Seven Day Yield, Caption [Text]	rr_MoneyMarketSevenDayYieldCaption	The 7-day yield for Shares of the Portfolio as of December 31, 2017
Money Market Seven Day Yield Phone	rr_MoneyMarketSevenDayYieldPhone	800-637-1380
Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT PORTFOLIO | Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.23%	[1]
Other Expenses	rr_OtherExpensesOverAssets	0.04%	[1]
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%	[1]
Service Fees	rr_Component2OtherExpensesOverAssets	none	[1]
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.02%	[1]
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.27%	[1]
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.02%)	[1],[2]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.25%	[1]
1 Year	rr_ExpenseExampleYear01	$ 26
3 Years	rr_ExpenseExampleYear03	85
5 Years	rr_ExpenseExampleYear05	150
10 Years	rr_ExpenseExampleYear10	$ 341
2008	rr_AnnualReturn2008	2.00%
2009	rr_AnnualReturn2009	0.07%
2010	rr_AnnualReturn2010	0.02%
2011	rr_AnnualReturn2011	0.02%
2012	rr_AnnualReturn2012	0.01%
2013	rr_AnnualReturn2013	0.01%
2014	rr_AnnualReturn2014	0.01%
2015	rr_AnnualReturn2015	0.01%
2016	rr_AnnualReturn2016	0.13%
2017	rr_AnnualReturn2017	0.68%
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to date total return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest quarterly return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.80%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest quarterly return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2015
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	none
1-Year	rr_AverageAnnualReturnYear01	0.68%
5-Year	rr_AverageAnnualReturnYear05	0.17%
10-Year	rr_AverageAnnualReturnYear10	0.29%
Since Inception	rr_AverageAnnualReturnSinceInception	3.40%
Inception Date	rr_AverageAnnualReturnInceptionDate	Oct. 29, 1985
Money Market Seven Day Yield	rr_MoneyMarketSevenDayYield	1.09%

[1] The expense information has been restated to reflect current fees.

[2] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.25%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Service and Premier Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT PORTFOLIO

<b>U.S. GOVERNMENT PORTFOLIO—SERVICE SHARES AND PREMIER SHARES</b>

<b>INVESTMENT OBJECTIVE </b>

The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.

<b>FEES AND EXPENSES OF THE PORTFOLIO </b>

This table describes the fees and expenses that you may pay if you buy and hold Service Shares and Premier Shares of the Portfolio.

<b>Shareholder Fees </b>(fees paid directly from your investment)

Shareholder Fees - Service and Premier Shares - NORTHERN INSTITUTIONAL U.S. GOVERNMENT PORTFOLIO - USD ($)	Service Shares	Premier Shares
Shareholder Fees (fees paid directly from your investment)	none	none

<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year<br/>as a percentage of the value of your investment)

Annual Fund Operating Expenses - Service and Premier Shares - NORTHERN INSTITUTIONAL U.S. GOVERNMENT PORTFOLIO		Service Shares	Premier Shares
Management Fees	[1]	0.23%	0.23%
Other Expenses	[1]	0.29%	0.54%	[2]
Transfer Agent Fees	[1]	0.02%	0.02%
Service Fees	[1]	0.25%	0.50%
Other Operating Expenses	[1]	0.02%	0.02%
Total Annual Portfolio Operating Expenses	[1]	0.52%	0.77%
Expense Reimbursement	[1],[3]	(0.02%)	(0.02%)
Total Annual Portfolio Operating Expenses After Expense Reimbursement	[1]	0.50%	0.75%

<b>EXAMPLE </b>

The following Example is intended to help you compare the cost of investing in Service Shares and Premier Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example - Service and Premier Shares - NORTHERN INSTITUTIONAL U.S. GOVERNMENT PORTFOLIO - USD ($)	1 Year	3 Years	5 Years	10 Years
Service Shares	51	165	289	651
Premier Shares	77	244	426	952

<b>PRINCIPAL INVESTMENT STRATEGIES </b>

The Portfolio seeks to achieve its objective by investing, under normal circumstances, substantially all (and at least 99.5%) of its total assets in cash, securities issued or guaranteed as to principal and interest by the U.S. government or by a person controlled or supervised by and acting as an instrumentality of the U.S. government pursuant to authority granted by the Congress of the United States or any certificate of deposit of any of the foregoing, and repurchase agreements that are fully collateralized by cash or such securities. The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in U.S. government securities and repurchase agreements collateralized solely by U.S. government securities.

The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur.

The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.

<b>PRINCIPAL RISKS </b>

CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect.

CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders.

DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities.

INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities.

INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio.

LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would.

MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results.

MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets.

PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities.

STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV.

U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future.

You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.

<b>PORTFOLIO PERFORMANCE</b>

The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Service Shares from year to year. As of December 31, 2017 there were no Premier Shares outstanding for the Portfolio.

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

Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.

<b>CALENDAR YEAR TOTAL RETURN (SERVICE SHARES)*</b>

* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.73% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the third quarter of 2015.

<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)

Average Annual Total Returns	Inception Date	1-Year	5-Year	10-Year	Since Inception
Service and Premier Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT PORTFOLIO | Service Shares	Apr. 01, 1999	0.68%	0.17%	0.26%	1.58%

The 7-day yield for Service Shares of the Portfolio as of December 31, 2017: 1.09%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.

[1] The expense information has been restated to reflect current fees.

[2] “Other Expenses” are based on estimated amounts for the current fiscal year.

[3] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.25%. The “Total Annual Portfolio Operating Expenses After Expense Reimbursement” may be higher than the contractual limitation as a result of certain Portfolio expenses, including but not limited to service fees, that are not reimbursed. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Service and Premier Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>U.S. GOVERNMENT PORTFOLIO—SERVICE SHARES AND PREMIER SHARES</b>
Objective [Heading]	rr_ObjectiveHeading	<b>INVESTMENT OBJECTIVE </b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.
Expense [Heading]	rr_ExpenseHeading	<b>FEES AND EXPENSES OF THE PORTFOLIO </b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Service Shares and Premier Shares of the Portfolio.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<b>Shareholder Fees </b>(fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year<br/>as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	The expense information has been restated to reflect current fees.
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>EXAMPLE </b>
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is intended to help you compare the cost of investing in Service Shares and Premier Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	<b>PRINCIPAL INVESTMENT STRATEGIES </b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Portfolio seeks to achieve its objective by investing, under normal circumstances, substantially all (and at least 99.5%) of its total assets in cash, securities issued or guaranteed as to principal and interest by the U.S. government or by a person controlled or supervised by and acting as an instrumentality of the U.S. government pursuant to authority granted by the Congress of the United States or any certificate of deposit of any of the foregoing, and repurchase agreements that are fully collateralized by cash or such securities. The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in U.S. government securities and repurchase agreements collateralized solely by U.S. government securities. The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur. The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.
Risk [Heading]	rr_RiskHeading	<b>PRINCIPAL RISKS </b>
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect. CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders. DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities. INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities. INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio. LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results. MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets. PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities. STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV. U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future. You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.
Risk Lose Money [Text]	rr_RiskLoseMoney	<b>You could lose money by investing in the Portfolio. </b>
Risk Money Market Fund May Not Preserve Dollar [Text]	rr_RiskMoneyMarketFundMayNotPreserveDollar	<b>Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. </b>
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	<b> An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. </b>
Risk Money Market Fund Sponsor May Not Provide Support [Text]	rr_RiskMoneyMarketFundSponsorMayNotProvideSupport	<b>The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time. </b>
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>PORTFOLIO PERFORMANCE</b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Service Shares from year to year. As of December 31, 2017 there were no Premier Shares outstanding for the Portfolio. The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future. Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Service Shares from year to year.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-637-1380
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	northerntrust.com/institutional
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	<b>CALENDAR YEAR TOTAL RETURN (SERVICE SHARES)*</b>
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.73% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the third quarter of 2015.
Performance Table Heading	rr_PerformanceTableHeading	<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	The 7-day yield for Service Shares of the Portfolio as of December 31, 2017: 1.09%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.
Money Market Seven Day Yield, Caption [Text]	rr_MoneyMarketSevenDayYieldCaption	The 7-day yield for Service Shares of the Portfolio as of December 31, 2017
Money Market Seven Day Yield Phone	rr_MoneyMarketSevenDayYieldPhone	800-637-1380
Service and Premier Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT PORTFOLIO | Service Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.23%	[1]
Other Expenses	rr_OtherExpensesOverAssets	0.29%	[1]
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%	[1]
Service Fees	rr_Component2OtherExpensesOverAssets	0.25%	[1]
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.02%	[1]
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.52%	[1]
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.02%)	[1],[2]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.50%	[1]
1 Year	rr_ExpenseExampleYear01	$ 51
3 Years	rr_ExpenseExampleYear03	165
5 Years	rr_ExpenseExampleYear05	289
10 Years	rr_ExpenseExampleYear10	$ 651
2008	rr_AnnualReturn2008	1.74%
2009	rr_AnnualReturn2009	0.03%
2010	rr_AnnualReturn2010	0.02%
2011	rr_AnnualReturn2011	0.02%
2012	rr_AnnualReturn2012	0.01%
2013	rr_AnnualReturn2013	0.01%
2014	rr_AnnualReturn2014	0.01%
2015	rr_AnnualReturn2015	0.01%
2016	rr_AnnualReturn2016	0.13%
2017	rr_AnnualReturn2017	0.68%
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to date total return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest quarterly return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.73%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest quarterly return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2015
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	none
1-Year	rr_AverageAnnualReturnYear01	0.68%
5-Year	rr_AverageAnnualReturnYear05	0.17%
10-Year	rr_AverageAnnualReturnYear10	0.26%
Since Inception	rr_AverageAnnualReturnSinceInception	1.58%
Inception Date	rr_AverageAnnualReturnInceptionDate	Apr. 01, 1999
Money Market Seven Day Yield	rr_MoneyMarketSevenDayYield	1.09%
Service and Premier Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT PORTFOLIO | Premier Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.23%	[1]
Other Expenses	rr_OtherExpensesOverAssets	0.54%	[1],[3]
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%	[1]
Service Fees	rr_Component2OtherExpensesOverAssets	0.50%	[1]
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.02%	[1]
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.77%	[1]
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.02%)	[1],[2]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.75%	[1]
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other Expenses” are based on estimated amounts for the current fiscal year.
1 Year	rr_ExpenseExampleYear01	$ 77
3 Years	rr_ExpenseExampleYear03	244
5 Years	rr_ExpenseExampleYear05	426
10 Years	rr_ExpenseExampleYear10	$ 952

[1] The expense information has been restated to reflect current fees.

[2] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.25%. The “Total Annual Portfolio Operating Expenses After Expense Reimbursement” may be higher than the contractual limitation as a result of certain Portfolio expenses, including but not limited to service fees, that are not reimbursed. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

[3] “Other Expenses” are based on estimated amounts for the current fiscal year.

XML

Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO

<b>U.S. GOVERNMENT SELECT PORTFOLIO—SHARES</b>

<b>INVESTMENT OBJECTIVE </b>

The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.

<b>FEES AND EXPENSES OF THE PORTFOLIO </b>

This table describes the fees and expenses that you may pay if you buy and hold Shares of the Portfolio.

<b>Shareholder Fees </b>(fees paid directly from your investment)

Shareholder Fees	Shares NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO Shares USD ($)
Shareholder Fees (fees paid directly from your investment)	none

<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)

Annual Fund Operating Expenses	Shares NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO Shares
Management Fees	0.18%
Other Expenses	0.03%
Transfer Agent Fees	0.02%
Service Fees	none
Other Operating Expenses	0.01%
Total Annual Portfolio Operating Expenses	0.21%
Expense Reimbursement	(0.01%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	0.20%

<b>EXAMPLE </b>

The following Example is intended to help you compare the cost of investing in Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example	1 Year	3 Years	5 Years	10 Years
Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO | Shares | USD ($)	20	67	117	267

<b>PRINCIPAL INVESTMENT STRATEGIES </b>

The Portfolio seeks to achieve its objective by investing, under normal circumstances, substantially all (and at least 99.5%) of its total assets in cash, securities issued or guaranteed as to principal and interest by the U.S. government or by a person controlled or supervised by and acting as an instrumentality of the U.S. government pursuant to authority granted by the Congress of the United States or any certificate of deposit of any of the foregoing, and repurchase agreements that are fully collateralized by cash or such securities. Subject to the foregoing 99.5% investment strategy requirement, under normal circumstances, the Portfolio will seek to acquire only those U.S. government securities paying interest that generally is exempt from state income taxation. The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in U.S. government securities and repurchase agreements collateralized solely by U.S. government securities.

The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur.

The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.

<b>PRINCIPAL RISKS </b>

CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect.

CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders.

DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities.

INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities.

INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio.

LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would.

MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results.

MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets.

PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities.

STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV.

U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future.

You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.

<b>PORTFOLIO PERFORMANCE </b>

The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Shares from year to year.

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

Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.

<b>CALENDAR YEAR TOTAL RETURN (SHARES)*</b>

* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.78% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the third quarter of 2015.

<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)

Average Annual Total Returns	Inception Date	1-Year	5-Year	10-Year	Since Inception
Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO | Shares	Nov. 07, 1990	0.73%	0.20%	0.31%	2.73%

The 7-day yield for Shares of the Portfolio as of December 31, 2017: 1.13%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.

[1] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.20%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>U.S. GOVERNMENT SELECT PORTFOLIO—SHARES</b>
Objective [Heading]	rr_ObjectiveHeading	<b>INVESTMENT OBJECTIVE </b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.
Expense [Heading]	rr_ExpenseHeading	<b>FEES AND EXPENSES OF THE PORTFOLIO </b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Shares of the Portfolio.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<b>Shareholder Fees </b>(fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>EXAMPLE </b>
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is intended to help you compare the cost of investing in Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	<b>PRINCIPAL INVESTMENT STRATEGIES </b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Portfolio seeks to achieve its objective by investing, under normal circumstances, substantially all (and at least 99.5%) of its total assets in cash, securities issued or guaranteed as to principal and interest by the U.S. government or by a person controlled or supervised by and acting as an instrumentality of the U.S. government pursuant to authority granted by the Congress of the United States or any certificate of deposit of any of the foregoing, and repurchase agreements that are fully collateralized by cash or such securities. Subject to the foregoing 99.5% investment strategy requirement, under normal circumstances, the Portfolio will seek to acquire only those U.S. government securities paying interest that generally is exempt from state income taxation. The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in U.S. government securities and repurchase agreements collateralized solely by U.S. government securities. The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur. The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.
Risk [Heading]	rr_RiskHeading	<b>PRINCIPAL RISKS </b>
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect. CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders. DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities. INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities. INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio. LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results. MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets. PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities. STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV. U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future. You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.
Risk Lose Money [Text]	rr_RiskLoseMoney	<b>You could lose money by investing in the Portfolio. </b>
Risk Money Market Fund May Not Preserve Dollar [Text]	rr_RiskMoneyMarketFundMayNotPreserveDollar	<b>Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. </b>
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	<b>An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. </b>
Risk Money Market Fund Sponsor May Not Provide Support [Text]	rr_RiskMoneyMarketFundSponsorMayNotProvideSupport	<b>The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time. </b>
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>PORTFOLIO PERFORMANCE </b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Shares from year to year. The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future. Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Shares from year to year.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-637-1380
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	northerntrust.com/institutional
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	<b>CALENDAR YEAR TOTAL RETURN (SHARES)*</b>
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.78% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the third quarter of 2015.
Performance Table Heading	rr_PerformanceTableHeading	<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	The 7-day yield for Shares of the Portfolio as of December 31, 2017: 1.13%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.
Money Market Seven Day Yield, Caption [Text]	rr_MoneyMarketSevenDayYieldCaption	The 7-day yield for Shares of the Portfolio as of December 31, 2017
Money Market Seven Day Yield Phone	rr_MoneyMarketSevenDayYieldPhone	800-637-1380
Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO | Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.18%
Other Expenses	rr_OtherExpensesOverAssets	0.03%
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	none
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.01%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.21%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.01%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.20%
1 Year	rr_ExpenseExampleYear01	$ 20
3 Years	rr_ExpenseExampleYear03	67
5 Years	rr_ExpenseExampleYear05	117
10 Years	rr_ExpenseExampleYear10	$ 267
2008	rr_AnnualReturn2008	1.99%
2009	rr_AnnualReturn2009	0.14%
2010	rr_AnnualReturn2010	0.03%
2011	rr_AnnualReturn2011	0.01%
2012	rr_AnnualReturn2012	0.01%
2013	rr_AnnualReturn2013	0.01%
2014	rr_AnnualReturn2014	0.01%
2015	rr_AnnualReturn2015	0.02%
2016	rr_AnnualReturn2016	0.23%
2017	rr_AnnualReturn2017	0.73%
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to date total return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest quarterly return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.78%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest quarterly return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2015
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	none
1-Year	rr_AverageAnnualReturnYear01	0.73%
5-Year	rr_AverageAnnualReturnYear05	0.20%
10-Year	rr_AverageAnnualReturnYear10	0.31%
Since Inception	rr_AverageAnnualReturnSinceInception	2.73%
Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 07, 1990
Money Market Seven Day Yield	rr_MoneyMarketSevenDayYield	1.13%

[1] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.20%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Service and Premier Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO

<b>U.S. GOVERNMENT SELECT PORTFOLIO—SERVICE SHARES AND PREMIER SHARES </b>

<b>INVESTMENT OBJECTIVE </b>

The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.

<b>FEES AND EXPENSES OF THE PORTFOLIO </b>

This table describes the fees and expenses that you may pay if you buy and hold Service Shares and Premier Shares of the Portfolio.

<b>Shareholder Fees </b>(fees paid directly from your investment)

Shareholder Fees - Service and Premier Shares - NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO - USD ($)	Service Shares	Premier Shares
Shareholder Fees (fees paid directly from your investment)	none	none

<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year<br/>as a percentage of the value of your investment)

Annual Fund Operating Expenses - Service and Premier Shares - NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO		Service Shares	Premier Shares
Management Fees		0.18%	0.18%
Other Expenses		0.28%	0.53%	[1]
Transfer Agent Fees		0.02%	0.02%
Service Fees		0.25%	0.50%
Other Operating Expenses		0.01%	0.01%
Total Annual Portfolio Operating Expenses		0.46%	0.71%
Expense Reimbursement	[2]	(0.01%)	(0.01%)
Total Annual Portfolio Operating Expenses After Expense Reimbursement		0.45%	0.70%

<b>EXAMPLE</b>

The following Example is intended to help you compare the cost of investing in the Service Shares and Premier Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example - Service and Premier Shares - NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO - USD ($)	1 Year	3 Years	5 Years	10 Years
Service Shares	46	147	257	578
Premier Shares	72	226	394	882

<b>PRINCIPAL INVESTMENT STRATEGIES </b>

The Portfolio seeks to achieve its objective by investing, under normal circumstances, substantially all (and at least 99.5%) of its total assets in cash, securities issued or guaranteed as to principal and interest by the U.S. government or by a person controlled or supervised by and acting as an instrumentality of the U.S. government pursuant to authority granted by the Congress of the United States or any certificate of deposit of any of the foregoing, and repurchase agreements that are fully collateralized by cash or such securities. Subject to the foregoing 99.5% investment strategy requirement, under normal circumstances, the Portfolio will seek to acquire only those U.S. government securities paying interest that generally is exempt from state income taxation. The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in U.S. government securities and repurchase agreements collateralized solely by U.S. government securities.

The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur.

The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.

<b>PRINCIPAL RISKS </b>

CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect.

CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders.

DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities.

INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities.

INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio.

LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would.

MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results.

MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets.

PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities.

STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV.

U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future.

You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.

<b>PORTFOLIO PERFORMANCE </b>

The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Service Shares from year to year. As of December 31, 2017 there were no Premier Shares outstanding for the Portfolio.

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

Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.

<b>CALENDAR YEAR TOTAL RETURN (SERVICE SHARES)* </b>

* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.72% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the third quarter of 2015.

<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)

Average Annual Total Returns	Inception Date	1-Year	5-Year	10-Year	Since Inception
Service and Premier Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO | Service Shares	May 28, 1999	0.72%	0.20%	0.28%	1.61%

The 7-day yield for Service Shares of the Portfolio as of December 31, 2017: 1.13%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.

[1] “Other Expenses” are based on estimated amounts for the current fiscal year.

[2] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.20%. The “Total Annual Portfolio Operating Expenses After Expense Reimbursement” may be higher than the contractual limitation as a result of certain Portfolio expenses, including but not limited to service fees, that are not reimbursed. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Service and Premier Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>U.S. GOVERNMENT SELECT PORTFOLIO—SERVICE SHARES AND PREMIER SHARES </b>
Objective [Heading]	rr_ObjectiveHeading	<b>INVESTMENT OBJECTIVE </b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.
Expense [Heading]	rr_ExpenseHeading	<b>FEES AND EXPENSES OF THE PORTFOLIO </b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Service Shares and Premier Shares of the Portfolio.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<b>Shareholder Fees </b>(fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year<br/>as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>EXAMPLE</b>
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is intended to help you compare the cost of investing in the Service Shares and Premier Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	<b>PRINCIPAL INVESTMENT STRATEGIES </b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Portfolio seeks to achieve its objective by investing, under normal circumstances, substantially all (and at least 99.5%) of its total assets in cash, securities issued or guaranteed as to principal and interest by the U.S. government or by a person controlled or supervised by and acting as an instrumentality of the U.S. government pursuant to authority granted by the Congress of the United States or any certificate of deposit of any of the foregoing, and repurchase agreements that are fully collateralized by cash or such securities. Subject to the foregoing 99.5% investment strategy requirement, under normal circumstances, the Portfolio will seek to acquire only those U.S. government securities paying interest that generally is exempt from state income taxation. The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in U.S. government securities and repurchase agreements collateralized solely by U.S. government securities. The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur. The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.
Risk [Heading]	rr_RiskHeading	<b>PRINCIPAL RISKS </b>
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect. CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders. DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities. INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities. INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio. LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results. MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets. PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities. STABLE NAV RISK is the risk that the Portfolio will not be able to maintain a NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV. U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future. You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.
Risk Lose Money [Text]	rr_RiskLoseMoney	<b>You could lose money by investing in the Portfolio.</b>
Risk Money Market Fund May Not Preserve Dollar [Text]	rr_RiskMoneyMarketFundMayNotPreserveDollar	<b>Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so.</b>
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	<b>An investment in the Portfolio is not a deposit of any bank and</b> <b>is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank.</b>
Risk Money Market Fund Sponsor May Not Provide Support [Text]	rr_RiskMoneyMarketFundSponsorMayNotProvideSupport	<b>The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.</b>
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>PORTFOLIO PERFORMANCE </b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Service Shares from year to year. As of December 31, 2017 there were no Premier Shares outstanding for the Portfolio. The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future. Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Service Shares from year to year.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-637-1380
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	northerntrust.com/institutional
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	<b>CALENDAR YEAR TOTAL RETURN (SERVICE SHARES)* </b>
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.72% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the third quarter of 2015.
Performance Table Heading	rr_PerformanceTableHeading	<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	The 7-day yield for Service Shares of the Portfolio as of December 31, 2017: 1.13%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.
Money Market Seven Day Yield, Caption [Text]	rr_MoneyMarketSevenDayYieldCaption	The 7-day yield for Service Shares of the Portfolio as of December 31, 2017
Money Market Seven Day Yield Phone	rr_MoneyMarketSevenDayYieldPhone	800-637-1380
Service and Premier Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO | Service Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.18%
Other Expenses	rr_OtherExpensesOverAssets	0.28%
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	0.25%
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.01%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.46%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.01%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.45%
1 Year	rr_ExpenseExampleYear01	$ 46
3 Years	rr_ExpenseExampleYear03	147
5 Years	rr_ExpenseExampleYear05	257
10 Years	rr_ExpenseExampleYear10	$ 578
2008	rr_AnnualReturn2008	1.73%
2009	rr_AnnualReturn2009	0.03%
2010	rr_AnnualReturn2010	0.02%
2011	rr_AnnualReturn2011	0.01%
2012	rr_AnnualReturn2012	0.01%
2013	rr_AnnualReturn2013	0.01%
2014	rr_AnnualReturn2014	0.01%
2015	rr_AnnualReturn2015	0.02%
2016	rr_AnnualReturn2016	0.23%
2017	rr_AnnualReturn2017	0.72%
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to date total return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest quarterly return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.72%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest quarterly return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2015
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	none
1-Year	rr_AverageAnnualReturnYear01	0.72%
5-Year	rr_AverageAnnualReturnYear05	0.20%
10-Year	rr_AverageAnnualReturnYear10	0.28%
Since Inception	rr_AverageAnnualReturnSinceInception	1.61%
Inception Date	rr_AverageAnnualReturnInceptionDate	May 28, 1999
Money Market Seven Day Yield	rr_MoneyMarketSevenDayYield	1.13%
Service and Premier Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO | Premier Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.18%
Other Expenses	rr_OtherExpensesOverAssets	0.53%	[2]
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	0.50%
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.01%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.71%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.01%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.70%
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other Expenses” are based on estimated amounts for the current fiscal year.
1 Year	rr_ExpenseExampleYear01	$ 72
3 Years	rr_ExpenseExampleYear03	226
5 Years	rr_ExpenseExampleYear05	394
10 Years	rr_ExpenseExampleYear10	$ 882

[1] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.20%. The “Total Annual Portfolio Operating Expenses After Expense Reimbursement” may be higher than the contractual limitation as a result of certain Portfolio expenses, including but not limited to service fees, that are not reimbursed. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

[2] “Other Expenses” are based on estimated amounts for the current fiscal year.

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Shares | NORTHERN INSTITUTIONAL MUNICIPAL PORTFOLIO

<b>MUNICIPAL PORTFOLIO—SHARES </b>

<b>INVESTMENT OBJECTIVE </b>

The Portfolio seeks to provide, to the extent consistent with the preservation of capital, a high level of income exempt from regular federal income tax by investing primarily in municipal instruments. This objective may be changed without shareholder approval.

<b>FEES AND EXPENSES OF THE PORTFOLIO </b>

This table describes the fees and expenses that you may pay if you buy and hold Shares of the Portfolio.

<b>Shareholder Fees </b>(fees paid directly from your investment)

Shareholder Fees	Shares NORTHERN INSTITUTIONAL MUNICIPAL PORTFOLIO Shares USD ($)
Shareholder Fees (fees paid directly from your investment)	none

<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)

Annual Fund Operating Expenses	Shares NORTHERN INSTITUTIONAL MUNICIPAL PORTFOLIO Shares
Management Fees	0.18%
Other Expenses	0.10%
Transfer Agent Fees	0.02%
Service Fees	none
Other Operating Expenses	0.08%
Total Annual Portfolio Operating Expenses	0.28%
Expense Reimbursement	(0.08%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	0.20%

<b>EXAMPLE </b>

The following Example is intended to help you compare the cost of investing in Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example	1 Year	3 Years	5 Years	10 Years
Shares | NORTHERN INSTITUTIONAL MUNICIPAL PORTFOLIO | Shares | USD ($)	20	82	149	348

<b>PRINCIPAL INVESTMENT STRATEGIES </b>

The Portfolio seeks to achieve its objective by investing primarily in high-quality short-term municipal instruments, the interest on which is exempt from regular federal income tax. The high level of income sought by the Portfolio is relative to yields currently available in the tax-exempt marketplace. Municipal instruments may include:

• Fixed, variable and floating rate notes and similar debt instruments;

• Asset-backed securities that are considered municipal instruments (such as trust certificates backed by municipal bonds);

• Tax-exempt commercial paper;

• Municipal bonds, notes, paper or other instruments; and

• Municipal bonds and notes that are guaranteed as to principal and interest or backed by the U.S. government or by its agencies, instrumentalities or sponsored enterprises.

Except in extraordinary circumstances, at least 80% of the Portfolio’s net assets will be invested in debt instruments that pay interest that is exempt from regular federal income tax. Alternative minimum tax (“AMT”) obligations, which pay interest that may be treated as an item of tax preference to shareholders under the federal AMT, will be deemed eligible investments for the purposes of determining whether the portfolio meets this policy. To the extent that the Portfolio invests in AMT obligations, a significant portion of the Portfolio’s dividends may be subject to federal income tax for shareholders subject to AMT. The Portfolio is not limited in the amount of its assets that may be invested in AMT obligations.

The Portfolio operates as an “institutional money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As an “institutional money market fund” under Rule 2a-7, the Portfolio is (1) required to sell and redeem its shares at its net asset value (“NAV”) rounded to the fourth decimal place (e.g., $1.0000) reflecting market-based values of the Portfolio’s holdings, and (2) the Board of Trustees (the “Board”) will be permitted to impose a “liquidity fee” on redemptions from the Portfolio (up to 2%) or temporarily restrict redemptions from the Portfolio for up to 10 business days during a 90-day period, as described in more detail under “Purchase and Sales of Portfolio Shares.”

The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.

<b>PRINCIPAL RISKS </b>

CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect.

CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders.

DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities.

FLOATING NAV RISK is the risk that because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.

GEOGRAPHIC AND SECTOR RISK is the risk that if the Portfolio invests a significant portion of its total assets in certain issuers within the same state, geographic region or economic sector, an adverse economic, business or political development affecting that state, region or sector may affect the value of the Portfolio’s investments more than if its investments were not so concentrated in such geographic region or economic sector.

GUARANTOR (OR CREDIT ENHANCEMENT) RISK is the risk that changes in credit quality of a U.S. or foreign bank, insurance company or other financial institution or such entity’s failure to fulfill its obligations could cause the Portfolio’s investments to decline in value. Adverse developments in the banking or bond insurance industries also may negatively affect the Portfolio.

INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities.

INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio.

LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would.

LIQUIDITY FEE AND REDEMPTION GATE RISK is the risk that the Portfolio may impose a “liquidity fee” (up to 2%) or “redemption gate” that temporarily restricts your ability to sell shares for up to 10 business days if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors.

LIQUIDITY RISK is the risk that the Portfolio will not be able to pay redemption proceeds in a timely manner because of unusual market conditions, an unusually high volume of redemption requests, legal restrictions impairing its ability to sell particular securities at an advantageous market price or other reasons. Certain portfolio securities may be less liquid than others, which may make them difficult or impossible to sell at the time and the price that the Portfolio would like and the Portfolio may have to lower the price, sell other securities instead or forgo an investment opportunity. Any of these events could have a negative effect on the Portfolio’s performance.

MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results.

MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets.

MUNICIPAL MARKET VOLATILITY RISK is the risk that the Portfolio may be adversely affected by the volatile municipal market. The municipal market can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities. In recent years an increasing number of municipal issuers have defaulted on obligations, been downgraded or commenced insolvency proceedings. Municipalities continue to experience difficulties in the current economic and political environment.

MUNICIPAL SECURITIES RISK is the risk that municipal securities may be subject to credit/default risk, interest rate risk and certain additional risks. The Portfolio may be more sensitive to adverse economic, business or political developments if it invests more than 25% of its assets in the debt securities of similar projects (such as those relating to education, health care, housing, transportation, and utilities), industrial development bonds, or in particular types of municipal securities (such as general obligation bonds, private activity bonds and moral obligation bonds). Municipal securities may be fully or partially backed or enhanced by the taxing authority of a local government, by the current or anticipated revenues from a specific project or specific assets, or by the credit of, or liquidity enhancement provided by, a private issuer. Special factors, such as legislative changes, and state and local economic and business developments, may adversely affect the yield and/or value of the Portfolio’s investments in municipal securities.

PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities.

TAX RISK is the risk that future legislative or administrative changes, court decisions or noncompliant conduct by issuers may materially affect the value of municipal instruments, the ability of the Portfolio to pay tax-exempt dividends, or may otherwise prevent the Portfolio from realizing the full current benefit of the tax-exempt status of such securities. Any such change could affect the value of an investment in the Portfolio.

VARIABLE OR FLOATING RATE INSTRUMENTS RISK is the risk that securities with variable or floating rates can be less sensitive to interest rate changes than securities with fixed interest rates, but may decline in value and negatively impact the Portfolio, particularly if changes in prevailing interest rates are more frequent or sudden than the rate changes for the variable or floating rate securities, which only occur periodically. Although variable and floating rate securities are less sensitive to interest rate risk than fixed-rate securities, they are subject to credit risk and default risk, which could impede their value.

You could lose money by investing in the Portfolio. Because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The Portfolio may impose a fee upon sale of your shares or may temporarily suspend your ability to sell shares if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.

<b>PORTFOLIO PERFORMANCE </b>

The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Shares from year to year.

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

Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.

<b>CALENDAR YEAR TOTAL RETURN (SHARES)* </b>

* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.63% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the third quarter of 2015.

<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)

Average Annual Total Returns	Inception Date	1-Year	5-Year	10-Year	Since Inception
Shares | NORTHERN INSTITUTIONAL MUNICIPAL PORTFOLIO | Shares	Dec. 01, 1999	0.63%	0.19%	0.36%	1.27%

The 7-day yield for Shares of the Portfolio as of December 31, 2017: 1.45%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.

[1] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.20%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Shares | NORTHERN INSTITUTIONAL MUNICIPAL PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>MUNICIPAL PORTFOLIO—SHARES </b>
Objective [Heading]	rr_ObjectiveHeading	<b>INVESTMENT OBJECTIVE </b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Portfolio seeks to provide, to the extent consistent with the preservation of capital, a high level of income exempt from regular federal income tax by investing primarily in municipal instruments. This objective may be changed without shareholder approval.
Expense [Heading]	rr_ExpenseHeading	<b>FEES AND EXPENSES OF THE PORTFOLIO </b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Shares of the Portfolio.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<b>Shareholder Fees </b>(fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>EXAMPLE </b>
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is intended to help you compare the cost of investing in Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	<b>PRINCIPAL INVESTMENT STRATEGIES </b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Portfolio seeks to achieve its objective by investing primarily in high-quality short-term municipal instruments, the interest on which is exempt from regular federal income tax. The high level of income sought by the Portfolio is relative to yields currently available in the tax-exempt marketplace. Municipal instruments may include: Fixed, variable and floating rate notes and similar debt instruments; Asset-backed securities that are considered municipal instruments (such as trust certificates backed by municipal bonds); Tax-exempt commercial paper; Municipal bonds, notes, paper or other instruments; and Municipal bonds and notes that are guaranteed as to principal and interest or backed by the U.S. government or by its agencies, instrumentalities or sponsored enterprises. Except in extraordinary circumstances, at least 80% of the Portfolio’s net assets will be invested in debt instruments that pay interest that is exempt from regular federal income tax. Alternative minimum tax (“AMT”) obligations, which pay interest that may be treated as an item of tax preference to shareholders under the federal AMT, will be deemed eligible investments for the purposes of determining whether the portfolio meets this policy. To the extent that the Portfolio invests in AMT obligations, a significant portion of the Portfolio’s dividends may be subject to federal income tax for shareholders subject to AMT. The Portfolio is not limited in the amount of its assets that may be invested in AMT obligations. The Portfolio operates as an “institutional money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As an “institutional money market fund” under Rule 2a-7, the Portfolio is (1) required to sell and redeem its shares at its net asset value (“NAV”) rounded to the fourth decimal place (e.g., $1.0000) reflecting market-based values of the Portfolio’s holdings, and (2) the Board of Trustees (the “Board”) will be permitted to impose a “liquidity fee” on redemptions from the Portfolio (up to 2%) or temporarily restrict redemptions from the Portfolio for up to 10 business days during a 90-day period, as described in more detail under “Purchase and Sales of Portfolio Shares.” The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.
Risk [Heading]	rr_RiskHeading	<b>PRINCIPAL RISKS </b>
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect. CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders. DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities. FLOATING NAV RISK is the risk that because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. GEOGRAPHIC AND SECTOR RISK is the risk that if the Portfolio invests a significant portion of its total assets in certain issuers within the same state, geographic region or economic sector, an adverse economic, business or political development affecting that state, region or sector may affect the value of the Portfolio’s investments more than if its investments were not so concentrated in such geographic region or economic sector. GUARANTOR (OR CREDIT ENHANCEMENT) RISK is the risk that changes in credit quality of a U.S. or foreign bank, insurance company or other financial institution or such entity’s failure to fulfill its obligations could cause the Portfolio’s investments to decline in value. Adverse developments in the banking or bond insurance industries also may negatively affect the Portfolio. INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities. INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio. LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. LIQUIDITY FEE AND REDEMPTION GATE RISK is the risk that the Portfolio may impose a “liquidity fee” (up to 2%) or “redemption gate” that temporarily restricts your ability to sell shares for up to 10 business days if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors. LIQUIDITY RISK is the risk that the Portfolio will not be able to pay redemption proceeds in a timely manner because of unusual market conditions, an unusually high volume of redemption requests, legal restrictions impairing its ability to sell particular securities at an advantageous market price or other reasons. Certain portfolio securities may be less liquid than others, which may make them difficult or impossible to sell at the time and the price that the Portfolio would like and the Portfolio may have to lower the price, sell other securities instead or forgo an investment opportunity. Any of these events could have a negative effect on the Portfolio’s performance. MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results. MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets. MUNICIPAL MARKET VOLATILITY RISK is the risk that the Portfolio may be adversely affected by the volatile municipal market. The municipal market can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities. In recent years an increasing number of municipal issuers have defaulted on obligations, been downgraded or commenced insolvency proceedings. Municipalities continue to experience difficulties in the current economic and political environment. MUNICIPAL SECURITIES RISK is the risk that municipal securities may be subject to credit/default risk, interest rate risk and certain additional risks. The Portfolio may be more sensitive to adverse economic, business or political developments if it invests more than 25% of its assets in the debt securities of similar projects (such as those relating to education, health care, housing, transportation, and utilities), industrial development bonds, or in particular types of municipal securities (such as general obligation bonds, private activity bonds and moral obligation bonds). Municipal securities may be fully or partially backed or enhanced by the taxing authority of a local government, by the current or anticipated revenues from a specific project or specific assets, or by the credit of, or liquidity enhancement provided by, a private issuer. Special factors, such as legislative changes, and state and local economic and business developments, may adversely affect the yield and/or value of the Portfolio’s investments in municipal securities. PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities. TAX RISK is the risk that future legislative or administrative changes, court decisions or noncompliant conduct by issuers may materially affect the value of municipal instruments, the ability of the Portfolio to pay tax-exempt dividends, or may otherwise prevent the Portfolio from realizing the full current benefit of the tax-exempt status of such securities. Any such change could affect the value of an investment in the Portfolio. VARIABLE OR FLOATING RATE INSTRUMENTS RISK is the risk that securities with variable or floating rates can be less sensitive to interest rate changes than securities with fixed interest rates, but may decline in value and negatively impact the Portfolio, particularly if changes in prevailing interest rates are more frequent or sudden than the rate changes for the variable or floating rate securities, which only occur periodically. Although variable and floating rate securities are less sensitive to interest rate risk than fixed-rate securities, they are subject to credit risk and default risk, which could impede their value. You could lose money by investing in the Portfolio. Because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The Portfolio may impose a fee upon sale of your shares or may temporarily suspend your ability to sell shares if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.
Risk Lose Money [Text]	rr_RiskLoseMoney	<b>You could lose money by investing in the Portfolio.</b>
Risk Money Market Fund Price Fluctuates [Text]	rr_RiskMoneyMarketFundPriceFluctuates	<b>Because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</b>
Risk Money Market Fund May Impose Fees or Suspend Sales [Text]	rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales	<b>The Portfolio may impose a fee upon sale of your shares or may temporarily suspend your ability to sell shares if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors.</b>
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	<b>An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank.</b>
Risk Money Market Fund Sponsor May Not Provide Support [Text]	rr_RiskMoneyMarketFundSponsorMayNotProvideSupport	<b>The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.</b>
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>PORTFOLIO PERFORMANCE </b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Shares from year to year. The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future. Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Shares from year to year.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-637-1380
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	northerntrust.com/institutional
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	<b>CALENDAR YEAR TOTAL RETURN (SHARES)* </b>
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.63% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the third quarter of 2015.
Performance Table Heading	rr_PerformanceTableHeading	<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	The 7-day yield for Shares of the Portfolio as of December 31, 2017: 1.45%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.
Money Market Seven Day Yield, Caption [Text]	rr_MoneyMarketSevenDayYieldCaption	The 7-day yield for Shares of the Portfolio as of December 31, 2017
Money Market Seven Day Yield Phone	rr_MoneyMarketSevenDayYieldPhone	800-637-1380
Shares | NORTHERN INSTITUTIONAL MUNICIPAL PORTFOLIO | Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.18%
Other Expenses	rr_OtherExpensesOverAssets	0.10%
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	none
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.08%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.28%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.08%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.20%
1 Year	rr_ExpenseExampleYear01	$ 20
3 Years	rr_ExpenseExampleYear03	82
5 Years	rr_ExpenseExampleYear05	149
10 Years	rr_ExpenseExampleYear10	$ 348
2008	rr_AnnualReturn2008	2.10%
2009	rr_AnnualReturn2009	0.42%
2010	rr_AnnualReturn2010	0.11%
2011	rr_AnnualReturn2011	0.04%
2012	rr_AnnualReturn2012	0.02%
2013	rr_AnnualReturn2013	0.02%
2014	rr_AnnualReturn2014	0.01%
2015	rr_AnnualReturn2015	0.02%
2016	rr_AnnualReturn2016	0.28%
2017	rr_AnnualReturn2017	0.63%
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to date total return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest quarterly return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.63%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest quarterly return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2015
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	none
1-Year	rr_AverageAnnualReturnYear01	0.63%
5-Year	rr_AverageAnnualReturnYear05	0.19%
10-Year	rr_AverageAnnualReturnYear10	0.36%
Since Inception	rr_AverageAnnualReturnSinceInception	1.27%
Inception Date	rr_AverageAnnualReturnInceptionDate	Dec. 01, 1999
Money Market Seven Day Yield	rr_MoneyMarketSevenDayYield	1.45%

[1] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.20%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

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Service and Premier Shares | NORTHERN INSTITUTIONAL MUNICIPAL PORTFOLIO

<b>MUNICIPAL PORTFOLIO—SERVICE SHARES AND PREMIER SHARES</b>

<b>INVESTMENT OBJECTIVE</b>

The Portfolio seeks to provide, to the extent consistent with the preservation of capital, a high level of income exempt from regular federal income tax by investing primarily in municipal instruments. This objective may be changed without shareholder approval.

<b>FEES AND EXPENSES OF THE PORTFOLIO</b>

This table describes the fees and expenses that you may pay if you buy and hold Service Shares and Premier Shares of the Portfolio.

<b>Shareholder Fees </b>(fees paid directly from your investment)

Shareholder Fees - Service and Premier Shares - NORTHERN INSTITUTIONAL MUNICIPAL PORTFOLIO - USD ($)	Service Shares	Premier Shares
Shareholder Fees (fees paid directly from your investment)	none	none

<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)

Annual Fund Operating Expenses - Service and Premier Shares - NORTHERN INSTITUTIONAL MUNICIPAL PORTFOLIO		Service Shares	Premier Shares
Management Fees		0.18%	0.18%
Other Expenses		0.35%	0.60%	[1]
Transfer Agent Fees		0.02%	0.02%
Service Fees		0.25%	0.50%
Other Operating Expenses		0.08%	0.08%
Total Annual Portfolio Operating Expenses		0.53%	0.78%
Expense Reimbursement	[2]	(0.08%)	(0.08%)
Total Annual Portfolio Operating Expenses After Expense Reimbursement		0.45%	0.70%

<b>EXAMPLE</b>

The following Example is intended to help you compare the cost of investing in Service Shares and Premier Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example - Service and Premier Shares - NORTHERN INSTITUTIONAL MUNICIPAL PORTFOLIO - USD ($)	1 Year	3 Years	5 Years	10 Years
Service Shares	46	162	288	657
Premier Shares	72	241	425	959

<b>PRINCIPAL INVESTMENT STRATEGIES</b>

The Portfolio seeks to achieve its objective by investing primarily in high-quality short-term municipal instruments, the interest on which is exempt from regular federal income tax. The high level of income sought by the Portfolio is relative to yields currently available in the tax-exempt marketplace. Municipal instruments may include:

• Fixed, variable and floating rate notes and similar debt instruments;

• Asset-backed securities that are considered municipal instruments (such as trust certificates backed by municipal bonds);

• Tax-exempt commercial paper;

• Municipal bonds, notes, paper or other instruments; and

• Municipal bonds and notes that are guaranteed as to principal and interest or backed by the U.S. government or by its agencies, instrumentalities or sponsored enterprises.

Except in extraordinary circumstances, at least 80% of the Portfolio’s net assets will be invested in debt instruments that pay interest that is exempt from regular federal income tax. Alternative minimum tax (“AMT”) obligations, which pay interest that may be treated as an item of tax preference to shareholders under the federal AMT, will be deemed eligible investments for the purposes of determining whether the portfolio meets this policy. To the extent that the Portfolio invests in AMT obligations, a significant portion of the Portfolio’s dividends may be subject to federal income tax for shareholders subject to AMT. The Portfolio is not limited in the amount of its assets that may be invested in AMT obligations.

The Portfolio operates as an “institutional money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As an “institutional money market fund” under Rule 2a-7, the Portfolio is (1) required to sell and redeem its shares at its net asset value (“NAV”) rounded to the fourth decimal place (e.g., $1.0000) reflecting market-based values of the Portfolio’s holdings, and (2) the Board of Trustees (the “Board”) will be permitted to impose a “liquidity fee” on redemptions from the Portfolio (up to 2%) or temporarily restrict redemptions from the Portfolio for up to 10 business days during a 90-day period, as described in more detail under “Purchase and Sales of Portfolio Shares.”

The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.

<b>PRINCIPAL RISKS</b>

CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect.

CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders.

DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities.

FLOATING NAV RISK is the risk that because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.

GEOGRAPHIC AND SECTOR RISK is the risk that if the Portfolio invests a significant portion of its total assets in certain issuers within the same state, geographic region or economic sector, an adverse economic, business or political development affecting that state, region or sector may affect the value of the Portfolio’s investments more than if its investments were not so concentrated in such geographic region or economic sector.

GUARANTOR (OR CREDIT ENHANCEMENT) RISK is the risk that changes in credit quality of a U.S. or foreign bank, insurance company or other financial institution or such entity’s failure to fulfill its obligations could cause the Portfolio’s investments to decline in value. Adverse developments in the banking or bond insurance industries also may negatively affect the Portfolio.

INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities.

INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio.

LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would.

LIQUIDITY FEE AND REDEMPTION GATE RISK is the risk that the Portfolio may impose a “liquidity fee” (up to 2%) or “redemption gate” that temporarily restricts your ability to sell shares for up to 10 business days if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors.

LIQUIDITY RISK is the risk that the Portfolio will not be able to pay redemption proceeds in a timely manner because of unusual market conditions, an unusually high volume of redemption requests, legal restrictions impairing its ability to sell particular securities at an advantageous market price or other reasons. Certain portfolio securities may be less liquid than others, which may make them difficult or impossible to sell at the time and the price that the Portfolio would like and the Portfolio may have to lower the price, sell other securities instead or forgo an investment opportunity. Any of these events could have a negative effect on the Portfolio’s performance.

MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results.

MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets.

MUNICIPAL MARKET VOLATILITY RISK is the risk that the Portfolio may be adversely affected by the volatile municipal market. The municipal market can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities. In recent years an increasing number of municipal issuers have defaulted on obligations, been downgraded or commenced insolvency proceedings. Municipalities continue to experience difficulties in the current economic and political environment.

MUNICIPAL SECURITIES RISK is the risk that municipal securities may be subject to credit/default risk, interest rate risk and certain additional risks. The Portfolio may be more sensitive to adverse economic, business or political developments if it invests more than 25% of its assets in the debt securities of similar projects (such as those relating to education, health care, housing, transportation, and utilities), industrial development bonds, or in particular types of municipal securities (such as general obligation bonds, private activity bonds and moral obligation bonds). Municipal securities may be fully or partially backed or enhanced by the taxing authority of a local government, by the current or anticipated revenues from a specific project or specific assets, or by the credit of, or liquidity enhancement provided by, a private issuer. Special factors, such as legislative changes, and state and local economic and business developments, may adversely affect the yield and/or value of the Portfolio’s investments in municipal securities.

PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities.

TAX RISK is the risk that future legislative or administrative changes, court decisions or noncompliant conduct by issuers may materially affect the value of municipal instruments, the ability of the Portfolio to pay tax-exempt dividends, or may otherwise prevent the Portfolio from realizing the full current benefit of the tax-exempt status of such securities. Any such change could affect the value of an investment in the Portfolio.

VARIABLE OR FLOATING RATE INSTRUMENTS RISK is the risk that securities with variable or floating rates can be less sensitive to interest rate changes than securities with fixed interest rates, but may decline in value and negatively impact the Portfolio, particularly if changes in prevailing interest rates are more frequent or sudden than the rate changes for the variable or floating rate securities, which only occur periodically. Although variable and floating rate securities are less sensitive to interest rate risk than fixed-rate securities, they are subject to credit risk and default risk, which could impede their value.

You could lose money by investing in the Portfolio. Because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The Portfolio may impose a fee upon sale of your shares or may temporarily suspend your ability to sell shares if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.

<b>PORTFOLIO PERFORMANCE</b>

The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Service Shares from year to year. As of December 31, 2017 there were no Premier Shares outstanding for the Portfolio.

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

Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.

<b>CALENDAR YEAR TOTAL RETURN (SERVICE SHARES)*</b>

* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.57% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the third quarter of 2015.

<b>AVERAGE ANNUAL TOTAL RETURNS</b><br/>(For the periods ended December 31, 2017)

Average Annual Total Returns	Inception Date	1-Year	5-Year	10-Year	Since Inception
Service and Premier Shares | NORTHERN INSTITUTIONAL MUNICIPAL PORTFOLIO | Service Shares	Feb. 11, 2000	0.63%	0.19%	0.30%	1.09%

The 7-day yield for Service Shares of the Portfolio as of December 31, 2017: 1.45%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.

[1] “Other Expenses” are based on estimated amounts for the current fiscal year.

[2] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.20%. The “Total Annual Portfolio Operating Expenses After Expense Reimbursement” may be higher than the contractual limitation as a result of certain Portfolio expenses, including but not limited to service fees, that are not reimbursed. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Service and Premier Shares | NORTHERN INSTITUTIONAL MUNICIPAL PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>MUNICIPAL PORTFOLIO—SERVICE SHARES AND PREMIER SHARES</b>
Objective [Heading]	rr_ObjectiveHeading	<b>INVESTMENT OBJECTIVE</b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Portfolio seeks to provide, to the extent consistent with the preservation of capital, a high level of income exempt from regular federal income tax by investing primarily in municipal instruments. This objective may be changed without shareholder approval.
Expense [Heading]	rr_ExpenseHeading	<b>FEES AND EXPENSES OF THE PORTFOLIO</b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Service Shares and Premier Shares of the Portfolio.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<b>Shareholder Fees </b>(fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>EXAMPLE</b>
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is intended to help you compare the cost of investing in Service Shares and Premier Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	<b>PRINCIPAL INVESTMENT STRATEGIES</b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Portfolio seeks to achieve its objective by investing primarily in high-quality short-term municipal instruments, the interest on which is exempt from regular federal income tax. The high level of income sought by the Portfolio is relative to yields currently available in the tax-exempt marketplace. Municipal instruments may include: Fixed, variable and floating rate notes and similar debt instruments; Asset-backed securities that are considered municipal instruments (such as trust certificates backed by municipal bonds); Tax-exempt commercial paper; Municipal bonds, notes, paper or other instruments; and Municipal bonds and notes that are guaranteed as to principal and interest or backed by the U.S. government or by its agencies, instrumentalities or sponsored enterprises. Except in extraordinary circumstances, at least 80% of the Portfolio’s net assets will be invested in debt instruments that pay interest that is exempt from regular federal income tax. Alternative minimum tax (“AMT”) obligations, which pay interest that may be treated as an item of tax preference to shareholders under the federal AMT, will be deemed eligible investments for the purposes of determining whether the portfolio meets this policy. To the extent that the Portfolio invests in AMT obligations, a significant portion of the Portfolio’s dividends may be subject to federal income tax for shareholders subject to AMT. The Portfolio is not limited in the amount of its assets that may be invested in AMT obligations. The Portfolio operates as an “institutional money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As an “institutional money market fund” under Rule 2a-7, the Portfolio is (1) required to sell and redeem its shares at its net asset value (“NAV”) rounded to the fourth decimal place (e.g., $1.0000) reflecting market-based values of the Portfolio’s holdings, and (2) the Board of Trustees (the “Board”) will be permitted to impose a “liquidity fee” on redemptions from the Portfolio (up to 2%) or temporarily restrict redemptions from the Portfolio for up to 10 business days during a 90-day period, as described in more detail under “Purchase and Sales of Portfolio Shares.” The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.
Risk [Heading]	rr_RiskHeading	<b>PRINCIPAL RISKS</b>
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect. CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders. DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities. FLOATING NAV RISK is the risk that because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. GEOGRAPHIC AND SECTOR RISK is the risk that if the Portfolio invests a significant portion of its total assets in certain issuers within the same state, geographic region or economic sector, an adverse economic, business or political development affecting that state, region or sector may affect the value of the Portfolio’s investments more than if its investments were not so concentrated in such geographic region or economic sector. GUARANTOR (OR CREDIT ENHANCEMENT) RISK is the risk that changes in credit quality of a U.S. or foreign bank, insurance company or other financial institution or such entity’s failure to fulfill its obligations could cause the Portfolio’s investments to decline in value. Adverse developments in the banking or bond insurance industries also may negatively affect the Portfolio. INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities. INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio. LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. LIQUIDITY FEE AND REDEMPTION GATE RISK is the risk that the Portfolio may impose a “liquidity fee” (up to 2%) or “redemption gate” that temporarily restricts your ability to sell shares for up to 10 business days if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors. LIQUIDITY RISK is the risk that the Portfolio will not be able to pay redemption proceeds in a timely manner because of unusual market conditions, an unusually high volume of redemption requests, legal restrictions impairing its ability to sell particular securities at an advantageous market price or other reasons. Certain portfolio securities may be less liquid than others, which may make them difficult or impossible to sell at the time and the price that the Portfolio would like and the Portfolio may have to lower the price, sell other securities instead or forgo an investment opportunity. Any of these events could have a negative effect on the Portfolio’s performance. MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results. MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets. MUNICIPAL MARKET VOLATILITY RISK is the risk that the Portfolio may be adversely affected by the volatile municipal market. The municipal market can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities. In recent years an increasing number of municipal issuers have defaulted on obligations, been downgraded or commenced insolvency proceedings. Municipalities continue to experience difficulties in the current economic and political environment. MUNICIPAL SECURITIES RISK is the risk that municipal securities may be subject to credit/default risk, interest rate risk and certain additional risks. The Portfolio may be more sensitive to adverse economic, business or political developments if it invests more than 25% of its assets in the debt securities of similar projects (such as those relating to education, health care, housing, transportation, and utilities), industrial development bonds, or in particular types of municipal securities (such as general obligation bonds, private activity bonds and moral obligation bonds). Municipal securities may be fully or partially backed or enhanced by the taxing authority of a local government, by the current or anticipated revenues from a specific project or specific assets, or by the credit of, or liquidity enhancement provided by, a private issuer. Special factors, such as legislative changes, and state and local economic and business developments, may adversely affect the yield and/or value of the Portfolio’s investments in municipal securities. PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities. TAX RISK is the risk that future legislative or administrative changes, court decisions or noncompliant conduct by issuers may materially affect the value of municipal instruments, the ability of the Portfolio to pay tax-exempt dividends, or may otherwise prevent the Portfolio from realizing the full current benefit of the tax-exempt status of such securities. Any such change could affect the value of an investment in the Portfolio. VARIABLE OR FLOATING RATE INSTRUMENTS RISK is the risk that securities with variable or floating rates can be less sensitive to interest rate changes than securities with fixed interest rates, but may decline in value and negatively impact the Portfolio, particularly if changes in prevailing interest rates are more frequent or sudden than the rate changes for the variable or floating rate securities, which only occur periodically. Although variable and floating rate securities are less sensitive to interest rate risk than fixed-rate securities, they are subject to credit risk and default risk, which could impede their value. You could lose money by investing in the Portfolio. Because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The Portfolio may impose a fee upon sale of your shares or may temporarily suspend your ability to sell shares if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.
Risk Lose Money [Text]	rr_RiskLoseMoney	<b>You could lose money by investing in the Portfolio.</b>
Risk Money Market Fund Price Fluctuates [Text]	rr_RiskMoneyMarketFundPriceFluctuates	<b>Because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</b>
Risk Money Market Fund May Impose Fees or Suspend Sales [Text]	rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales	<b>The Portfolio may impose a fee upon sale of your shares or may temporarily suspend your ability to sell shares if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors.</b>
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	<b>An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank.</b>
Risk Money Market Fund Sponsor May Not Provide Support [Text]	rr_RiskMoneyMarketFundSponsorMayNotProvideSupport	<b>The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.</b>
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>PORTFOLIO PERFORMANCE</b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Service Shares from year to year. As of December 31, 2017 there were no Premier Shares outstanding for the Portfolio. The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future. Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Service Shares from year to year.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-637-1380
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	northerntrust.com/institutional
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	<b>CALENDAR YEAR TOTAL RETURN (SERVICE SHARES)*</b>
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.57% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the third quarter of 2015.
Performance Table Heading	rr_PerformanceTableHeading	<b>AVERAGE ANNUAL TOTAL RETURNS</b><br/>(For the periods ended December 31, 2017)
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	The 7-day yield for Service Shares of the Portfolio as of December 31, 2017: 1.45%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.
Money Market Seven Day Yield, Caption [Text]	rr_MoneyMarketSevenDayYieldCaption	The 7-day yield for Service Shares of the Portfolio as of December 31, 2017
Money Market Seven Day Yield Phone	rr_MoneyMarketSevenDayYieldPhone	800-637-1380
Service and Premier Shares | NORTHERN INSTITUTIONAL MUNICIPAL PORTFOLIO | Service Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.18%
Other Expenses	rr_OtherExpensesOverAssets	0.35%
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	0.25%
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.08%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.53%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.08%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.45%
1 Year	rr_ExpenseExampleYear01	$ 46
3 Years	rr_ExpenseExampleYear03	162
5 Years	rr_ExpenseExampleYear05	288
10 Years	rr_ExpenseExampleYear10	$ 657
2008	rr_AnnualReturn2008	1.83%
2009	rr_AnnualReturn2009	0.19%
2010	rr_AnnualReturn2010	0.02%
2011	rr_AnnualReturn2011	0.02%
2012	rr_AnnualReturn2012	0.02%
2013	rr_AnnualReturn2013	0.02%
2014	rr_AnnualReturn2014	0.01%
2015	rr_AnnualReturn2015	0.02%
2016	rr_AnnualReturn2016	0.28%
2017	rr_AnnualReturn2017	0.63%
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to date total return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest quarterly return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.57%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest quarterly return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2015
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	none
1-Year	rr_AverageAnnualReturnYear01	0.63%
5-Year	rr_AverageAnnualReturnYear05	0.19%
10-Year	rr_AverageAnnualReturnYear10	0.30%
Since Inception	rr_AverageAnnualReturnSinceInception	1.09%
Inception Date	rr_AverageAnnualReturnInceptionDate	Feb. 11, 2000
Money Market Seven Day Yield	rr_MoneyMarketSevenDayYield	1.45%
Service and Premier Shares | NORTHERN INSTITUTIONAL MUNICIPAL PORTFOLIO | Premier Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.18%
Other Expenses	rr_OtherExpensesOverAssets	0.60%	[2]
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	0.50%
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.08%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.78%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.08%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.70%
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other Expenses” are based on estimated amounts for the current fiscal year.
1 Year	rr_ExpenseExampleYear01	$ 72
3 Years	rr_ExpenseExampleYear03	241
5 Years	rr_ExpenseExampleYear05	425
10 Years	rr_ExpenseExampleYear10	$ 959

[1] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.20%. The “Total Annual Portfolio Operating Expenses After Expense Reimbursement” may be higher than the contractual limitation as a result of certain Portfolio expenses, including but not limited to service fees, that are not reimbursed. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

[2] “Other Expenses” are based on estimated amounts for the current fiscal year.

XML

Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO

<b>PRIME OBLIGATIONS PORTFOLIO—SHARES</b>

<b>INVESTMENT OBJECTIVE</b>

The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.

<b>FEES AND EXPENSES OF THE PORTFOLIO</b>

This table describes the fees and expenses that you may pay if you buy and hold Shares of the Portfolio.

<b>Shareholder Fees </b>(fees paid directly from your investment)

Shareholder Fees	Shares NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO Shares USD ($)
Shareholder Fees (fees paid directly from your investment)	none

<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)

Annual Fund Operating Expenses	Shares NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO Shares
Management Fees	0.13%
Other Expenses	0.04%
Transfer Agent Fees	0.02%
Service Fees	none
Other Operating Expenses	0.02%
Total Annual Portfolio Operating Expenses	0.17%
Expense Reimbursement	(0.02%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	0.15%

<b>EXAMPLE</b>

The following Example is intended to help you compare the cost of investing in Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example	1 Year	3 Years	5 Years	10 Years
Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO | Shares | USD ($)	15	53	94	215

<b>PRINCIPAL INVESTMENT STRATEGIES</b>

The Portfolio seeks to achieve its objective by investing in a broad range of high-quality, U.S. dollar-denominated government, bank and commercial obligations that are available in the money markets, including:

• Obligations of U.S. banks (including obligations of foreign branches of such banks);

• Obligations of foreign commercial banks;

• Commercial paper and other obligations issued or guaranteed by U.S. and foreign corporations and other issuers;

• Corporate bonds, notes, paper and other instruments that are of high quality;

• Asset-backed securities and asset-backed commercial paper;

• Securities issued or guaranteed as to principal and interest by the U.S. government or by its agencies, instrumentalities or sponsored enterprises and custodial receipts with respect thereto;

• Securities issued or guaranteed by one or more foreign governments or political subdivisions, agencies or instrumentalities;

• Repurchase agreements; and

• Municipal securities issued or guaranteed by state or local governmental bodies.

Under normal market conditions, the Portfolio will invest at least 25% of its total assets in securities issued by companies in the financial services group of industries. Companies in the financial services group of industries include but are not limited to U.S. and non-U.S. companies involved in banking, mortgage, consumer or specialized finance, investment banking, securities brokerage, asset management and custody, insurance, financial investment, real estate and mortgage finance and financial conglomerates, and related asset-backed securities. The Portfolio may, however, for temporary defensive purposes, invest less than 25% of its total assets in the financial services industry if warranted due to adverse economic conditions or if investing less than 25% of its total assets in the financial services industry appears to be in the best interest of shareholders.

The Portfolio operates as an “institutional money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As an “institutional money market fund” under Rule 2a-7, the Portfolio is (1) required to sell and redeem its shares at its net asset value (“NAV”) rounded to the fourth decimal place (e.g., $1.0000) reflecting market-based values of the Portfolio’s holdings, and (2) the Board of Trustees (the “Board”) will be permitted to impose a “liquidity fee” on redemptions from the Portfolio (up to 2%) or temporarily restrict redemptions from the Portfolio for up to 10 business days during a 90-day period, as described in more detail under “Purchase and Sales of Portfolio Shares.”

The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.

<b>PRINCIPAL RISKS</b>

ASSET-BACKED SECURITIES RISK. Asset-backed securities represent interests in pools of assets such as mortgages, automobile loans, credit card receivables and other financial assets. Asset-backed securities are subject to credit, interest rate, prepayment, extension, valuation and liquidity risk. These securities, in most cases, are not backed by the full faith and credit of the U.S. government and are subject to the risk of default on the underlying asset or loan, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain asset-backed securities.

COMMERCIAL PAPER RISK. Investments in commercial paper are subject to the risk that the issuer cannot issue enough new commercial paper to satisfy its obligations with respect to its outstanding commercial paper, also known as rollover risk. Commercial paper is also susceptible to changes in the issuer’s financial condition or credit quality. In addition, under certain circumstances commercial paper may become illiquid or may suffer from reduced liquidity. Commercial paper is generally unsecured, which increases the credit risk associated with this type of investment.

CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect.

CURRENCY RISK is the risk that foreign currencies, securities that trade in or receive revenue in foreign currencies, or derivatives that provide exposure to foreign currencies will fluctuate in value relative to the U.S. dollar, adversely affecting the value of the Fund’s investments and its returns. Because the Portfolio’s NAV is determined on the basis of U.S. dollars, you may lose money if the local currency of a foreign market depreciates against the U.S. dollar, even if the market value of the Portfolio’s holdings appreciates. In addition, fluctuations in the exchange values of currencies could affect the economy or particular business operations of companies in a geographic region in which the Portfolio invests, causing an adverse impact on the Portfolio’s investments in the affected region.

CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders.

DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities.

FINANCIAL SERVICES INDUSTRY RISK is the risk that, because the Portfolio will invest under normal market conditions at least 25% of its total assets in the financial services industry, the Portfolio will be subject to greater risk of loss by economic, business, political or other developments which generally affect this industry. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

FLOATING NAV RISK is the risk that because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.

FOREIGN SECURITIES RISK is the risk that a foreign security, even if it is a U.S. dollar-denominated foreign security, could lose value as a result of political, financial and economic events in foreign countries, more or less stringent foreign securities regulations and accounting and disclosure standards or other factors. In addition, the Portfolio will be subject to the risk that an issuer of foreign sovereign debt or the government authorities that control the repayment of the debt may be unable or unwilling to repay the principal or interest when due.

GUARANTOR (OR CREDIT ENHANCEMENT) RISK is the risk that changes in credit quality of a U.S. or foreign bank, insurance company or other financial institution or such entity’s failure to fulfill its obligations could cause the Portfolio’s investments to decline in value. Adverse developments in the banking or bond insurance industries also may negatively affect the Portfolio.

INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities.

INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio.

LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would.

LIQUIDITY FEE AND REDEMPTION GATE RISK is the risk that the Portfolio may impose a “liquidity fee” (up to 2%) or “redemption gate” that temporarily restricts your ability to sell shares for up to 10 business days if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors.

LIQUIDITY RISK is the risk that the Portfolio will not be able to pay redemption proceeds in a timely manner because of unusual market conditions, an unusually high volume of redemption requests, legal restrictions impairing its ability to sell particular securities at an advantageous market price or other reasons. Certain portfolio securities may be less liquid than others, which may make them difficult or impossible to sell at the time and the price that the Portfolio would like and the Portfolio may have to lower the price, sell other securities instead or forgo an investment opportunity. Any of these events could have a negative effect on the Portfolio’s performance.

MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results.

MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets.

PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities.

U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future.

You could lose money by investing in the Portfolio. Because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The Portfolio may impose a fee upon sale of your shares or may temporarily suspend your ability to sell shares if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.

<b>PORTFOLIO PERFORMANCE</b>

The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Shares from year to year.

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

Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.

<b>CALENDAR YEAR TOTAL RETURN (SHARES)*</b>

* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.87% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the first quarter of 2014.

<b>AVERAGE ANNUAL TOTAL RETURNS</b><br/>(For the periods ended December 31, 2017)

Average Annual Total Returns	Inception Date	1-Year	5-Year	10-Year	Since Inception
Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO | Shares	Aug. 21, 2003	1.04%	0.34%	0.46%	1.34%

The 7-day yield for Shares of the Portfolio as of December 31, 2017: 1.41%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.

[1] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.15%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>PRIME OBLIGATIONS PORTFOLIO—SHARES</b>
Objective [Heading]	rr_ObjectiveHeading	<b>INVESTMENT OBJECTIVE</b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.
Expense [Heading]	rr_ExpenseHeading	<b>FEES AND EXPENSES OF THE PORTFOLIO</b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Shares of the Portfolio.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<b>Shareholder Fees </b>(fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>EXAMPLE</b>
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is intended to help you compare the cost of investing in Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	<b>PRINCIPAL INVESTMENT STRATEGIES</b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Portfolio seeks to achieve its objective by investing in a broad range of high-quality, U.S. dollar-denominated government, bank and commercial obligations that are available in the money markets, including: Obligations of U.S. banks (including obligations of foreign branches of such banks); Obligations of foreign commercial banks; Commercial paper and other obligations issued or guaranteed by U.S. and foreign corporations and other issuers; Corporate bonds, notes, paper and other instruments that are of high quality; Asset-backed securities and asset-backed commercial paper; Securities issued or guaranteed as to principal and interest by the U.S. government or by its agencies, instrumentalities or sponsored enterprises and custodial receipts with respect thereto; Securities issued or guaranteed by one or more foreign governments or political subdivisions, agencies or instrumentalities; Repurchase agreements; and Municipal securities issued or guaranteed by state or local governmental bodies. Under normal market conditions, the Portfolio will invest at least 25% of its total assets in securities issued by companies in the financial services group of industries. Companies in the financial services group of industries include but are not limited to U.S. and non-U.S. companies involved in banking, mortgage, consumer or specialized finance, investment banking, securities brokerage, asset management and custody, insurance, financial investment, real estate and mortgage finance and financial conglomerates, and related asset-backed securities. The Portfolio may, however, for temporary defensive purposes, invest less than 25% of its total assets in the financial services industry if warranted due to adverse economic conditions or if investing less than 25% of its total assets in the financial services industry appears to be in the best interest of shareholders. The Portfolio operates as an “institutional money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As an “institutional money market fund” under Rule 2a-7, the Portfolio is (1) required to sell and redeem its shares at its net asset value (“NAV”) rounded to the fourth decimal place (e.g., $1.0000) reflecting market-based values of the Portfolio’s holdings, and (2) the Board of Trustees (the “Board”) will be permitted to impose a “liquidity fee” on redemptions from the Portfolio (up to 2%) or temporarily restrict redemptions from the Portfolio for up to 10 business days during a 90-day period, as described in more detail under “Purchase and Sales of Portfolio Shares.” The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.
Risk [Heading]	rr_RiskHeading	<b>PRINCIPAL RISKS</b>
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	ASSET-BACKED SECURITIES RISK. Asset-backed securities represent interests in pools of assets such as mortgages, automobile loans, credit card receivables and other financial assets. Asset-backed securities are subject to credit, interest rate, prepayment, extension, valuation and liquidity risk. These securities, in most cases, are not backed by the full faith and credit of the U.S. government and are subject to the risk of default on the underlying asset or loan, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain asset-backed securities. COMMERCIAL PAPER RISK. Investments in commercial paper are subject to the risk that the issuer cannot issue enough new commercial paper to satisfy its obligations with respect to its outstanding commercial paper, also known as rollover risk. Commercial paper is also susceptible to changes in the issuer’s financial condition or credit quality. In addition, under certain circumstances commercial paper may become illiquid or may suffer from reduced liquidity. Commercial paper is generally unsecured, which increases the credit risk associated with this type of investment. CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect. CURRENCY RISK is the risk that foreign currencies, securities that trade in or receive revenue in foreign currencies, or derivatives that provide exposure to foreign currencies will fluctuate in value relative to the U.S. dollar, adversely affecting the value of the Fund’s investments and its returns. Because the Portfolio’s NAV is determined on the basis of U.S. dollars, you may lose money if the local currency of a foreign market depreciates against the U.S. dollar, even if the market value of the Portfolio’s holdings appreciates. In addition, fluctuations in the exchange values of currencies could affect the economy or particular business operations of companies in a geographic region in which the Portfolio invests, causing an adverse impact on the Portfolio’s investments in the affected region. CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders. DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities. FINANCIAL SERVICES INDUSTRY RISK is the risk that, because the Portfolio will invest under normal market conditions at least 25% of its total assets in the financial services industry, the Portfolio will be subject to greater risk of loss by economic, business, political or other developments which generally affect this industry. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations. FLOATING NAV RISK is the risk that because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. FOREIGN SECURITIES RISK is the risk that a foreign security, even if it is a U.S. dollar-denominated foreign security, could lose value as a result of political, financial and economic events in foreign countries, more or less stringent foreign securities regulations and accounting and disclosure standards or other factors. In addition, the Portfolio will be subject to the risk that an issuer of foreign sovereign debt or the government authorities that control the repayment of the debt may be unable or unwilling to repay the principal or interest when due. GUARANTOR (OR CREDIT ENHANCEMENT) RISK is the risk that changes in credit quality of a U.S. or foreign bank, insurance company or other financial institution or such entity’s failure to fulfill its obligations could cause the Portfolio’s investments to decline in value. Adverse developments in the banking or bond insurance industries also may negatively affect the Portfolio. INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities. INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio. LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. LIQUIDITY FEE AND REDEMPTION GATE RISK is the risk that the Portfolio may impose a “liquidity fee” (up to 2%) or “redemption gate” that temporarily restricts your ability to sell shares for up to 10 business days if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors. LIQUIDITY RISK is the risk that the Portfolio will not be able to pay redemption proceeds in a timely manner because of unusual market conditions, an unusually high volume of redemption requests, legal restrictions impairing its ability to sell particular securities at an advantageous market price or other reasons. Certain portfolio securities may be less liquid than others, which may make them difficult or impossible to sell at the time and the price that the Portfolio would like and the Portfolio may have to lower the price, sell other securities instead or forgo an investment opportunity. Any of these events could have a negative effect on the Portfolio’s performance. MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results. MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets. PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities. U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future. You could lose money by investing in the Portfolio. Because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The Portfolio may impose a fee upon sale of your shares or may temporarily suspend your ability to sell shares if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.
Risk Lose Money [Text]	rr_RiskLoseMoney	<b>You could lose money by investing in the Portfolio.</b>
Risk Money Market Fund Price Fluctuates [Text]	rr_RiskMoneyMarketFundPriceFluctuates	<b>Because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</b>
Risk Money Market Fund May Impose Fees or Suspend Sales [Text]	rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales	<b>The Portfolio may impose a fee upon sale of your shares or may temporarily suspend your ability to sell shares if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors.</b>
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	<b>An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank.</b>
Risk Money Market Fund Sponsor May Not Provide Support [Text]	rr_RiskMoneyMarketFundSponsorMayNotProvideSupport	<b>The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.</b>
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>PORTFOLIO PERFORMANCE</b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Shares from year to year. The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future. Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio’s Shares from year to year.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-637-1380
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	northerntrust.com/institutional
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	<b>CALENDAR YEAR TOTAL RETURN (SHARES)*</b>
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.87% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the first quarter of 2014.
Performance Table Heading	rr_PerformanceTableHeading	<b>AVERAGE ANNUAL TOTAL RETURNS</b><br/>(For the periods ended December 31, 2017)
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	The 7-day yield for Shares of the Portfolio as of December 31, 2017: 1.41%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.
Money Market Seven Day Yield, Caption [Text]	rr_MoneyMarketSevenDayYieldCaption	The 7-day yield for Shares of the Portfolio as of December 31, 2017
Money Market Seven Day Yield Phone	rr_MoneyMarketSevenDayYieldPhone	800-637-1380
Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO | Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.13%
Other Expenses	rr_OtherExpensesOverAssets	0.04%
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	none
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.02%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.17%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.02%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.15%
1 Year	rr_ExpenseExampleYear01	$ 15
3 Years	rr_ExpenseExampleYear03	53
5 Years	rr_ExpenseExampleYear05	94
10 Years	rr_ExpenseExampleYear10	$ 215
2008	rr_AnnualReturn2008	2.23%
2009	rr_AnnualReturn2009	0.39%
2010	rr_AnnualReturn2010	0.15%
2011	rr_AnnualReturn2011	0.08%
2012	rr_AnnualReturn2012	0.08%
2013	rr_AnnualReturn2013	0.05%
2014	rr_AnnualReturn2014	0.02%
2015	rr_AnnualReturn2015	0.07%
2016	rr_AnnualReturn2016	0.49%
2017	rr_AnnualReturn2017	1.04%
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to date total return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest quarterly return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.87%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest quarterly return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2014
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	none
1-Year	rr_AverageAnnualReturnYear01	1.04%
5-Year	rr_AverageAnnualReturnYear05	0.34%
10-Year	rr_AverageAnnualReturnYear10	0.46%
Since Inception	rr_AverageAnnualReturnSinceInception	1.34%
Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 21, 2003
Money Market Seven Day Yield	rr_MoneyMarketSevenDayYield	1.41%

[1] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.15%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Service and Premier Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO

<b>PRIME OBLIGATIONS PORTFOLIO—SERVICE SHARES AND PREMIER SHARES</b>

<b>INVESTMENT OBJECTIVE</b>

The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.

<b>FEES AND EXPENSES OF THE PORTFOLIO</b>

This table describes the fees and expenses that you may pay if you buy and hold Service Shares and Premier Shares of the Portfolio.

<b>Shareholder Fees </b>(fees paid directly from your investment)

Shareholder Fees - Service and Premier Shares - NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO - USD ($)	Service Shares	Premier Shares
Shareholder Fees (fees paid directly from your investment)	none	none

<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)

Annual Fund Operating Expenses - Service and Premier Shares - NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO		Service Shares	Premier Shares
Management Fees		0.13%	0.13%
Other Expenses	[1]	0.29%	0.54%
Transfer Agent Fees		0.02%	0.02%
Service Fees		0.25%	0.50%
Other Operating Expenses		0.02%	0.02%
Total Annual Portfolio Operating Expenses		0.42%	0.67%
Expense Reimbursement	[2]	(0.02%)	(0.02%)
Total Annual Portfolio Operating Expenses After Expense Reimbursement		0.40%	0.65%

<b>EXAMPLE</b>

The following Example is intended to help you compare the cost of investing in Service Shares and Premier Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example - Service and Premier Shares - NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO - USD ($)	1 Year	3 Years	5 Years	10 Years
Service Shares	41	133	233	528
Premier Shares	66	212	371	833

<b>PRINCIPAL INVESTMENT STRATEGIES</b>

The Portfolio seeks to achieve its objective by investing in a broad range of high-quality, U.S. dollar-denominated government, bank and commercial obligations that are available in the money markets, including:

• Obligations of U.S. banks (including obligations of foreign branches of such banks);

• Obligations of foreign commercial banks;

• Commercial paper and other obligations issued or guaranteed by U.S. and foreign corporations and other issuers;

• Corporate bonds, notes, paper and other instruments that are of high quality;

• Asset-backed securities and asset-backed commercial paper;

• Securities issued or guaranteed as to principal and interest by the U.S. government or by its agencies, instrumentalities or sponsored enterprises and custodial receipts with respect thereto;

• Securities issued or guaranteed by one or more foreign governments or political subdivisions, agencies or instrumentalities;

• Repurchase agreements; and

• Municipal securities issued or guaranteed by state or local governmental bodies.

Under normal market conditions, the Portfolio will invest at least 25% of its total assets in securities issued by companies in the financial services group of industries. Companies in the financial services group of industries include but are not limited to U.S. and non-U.S. companies involved in banking, mortgage, consumer or specialized finance, investment banking, securities brokerage, asset management and custody, insurance, financial investment, real estate and mortgage finance and financial conglomerates, and related asset-backed securities. The Portfolio may, however, for temporary defensive purposes, invest less than 25% of its total assets in the financial services industry if warranted due to adverse economic conditions or if investing less than 25% of its total assets in the financial services industry appears to be in the best interest of shareholders.

The Portfolio operates as an “institutional money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As an “institutional money market fund” under Rule 2a-7, the Portfolio is (1) required to sell and redeem its shares at its net asset value (“NAV”) rounded to the fourth decimal place (e.g., $1.0000) reflecting market-based values of the Portfolio’s holdings, and (2) the Board of Trustees (the “Board”) will be permitted to impose a “liquidity fee” on redemptions from the Portfolio (up to 2%) or temporarily restrict redemptions from the Portfolio for up to 10 business days during a 90-day period, as described in more detail under “Purchase and Sales of Portfolio Shares.”

The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments.

Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.

<b>PRINCIPAL RISKS</b>

ASSET-BACKED SECURITIES RISK. Asset-backed securities represent interests in pools of assets such as mortgages, automobile loans, credit card receivables and other financial assets. Asset-backed securities are subject to credit, interest rate, prepayment, extension, valuation and liquidity risk. These securities, in most cases, are not backed by the full faith and credit of the U.S. government and are subject to the risk of default on the underlying asset or loan, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain asset-backed securities.

COMMERCIAL PAPER RISK. Investments in commercial paper are subject to the risk that the issuer cannot issue enough new commercial paper to satisfy its obligations with respect to its outstanding commercial paper, also known as rollover risk. Commercial paper is also susceptible to changes in the issuer’s financial condition or credit quality. In addition, under certain circumstances commercial paper may become illiquid or may suffer from reduced liquidity. Commercial paper is generally unsecured, which increases the credit risk associated with this type of investment.

CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect.

CURRENCY RISK is the risk that foreign currencies, securities that trade in or receive revenue in foreign currencies, or derivatives that provide exposure to foreign currencies will fluctuate in value relative to the U.S. dollar, adversely affecting the value of the Fund’s investments and its returns. Because the Portfolio’s NAV is determined on the basis of U.S. dollars, you may lose money if the local currency of a foreign market depreciates against the U.S. dollar, even if the market value of the Portfolio’s holdings appreciates. In addition, fluctuations in the exchange values of currencies could affect the economy or particular business operations of companies in a geographic region in which the Portfolio invests, causing an adverse impact on the Portfolio’s investments in the affected region.

CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders.

DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities.

FINANCIAL SERVICES INDUSTRY RISK is the risk that, because the Portfolio will invest under normal market conditions at least 25% of its total assets in the financial services industry, the Portfolio will be subject to greater risk of loss by economic, business, political or other developments which generally affect this industry. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

FLOATING NAV RISK is the risk that because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.

FOREIGN SECURITIES RISK is the risk that a foreign security, even if it is a U.S. dollar-denominated foreign security, could lose value as a result of political, financial and economic events in foreign countries, more or less stringent foreign securities regulations and accounting and disclosure standards or other factors. In addition, the Portfolio will be subject to the risk that an issuer of foreign sovereign debt or the government authorities that control the repayment of the debt may be unable or unwilling to repay the principal or interest when due.

GUARANTOR (OR CREDIT ENHANCEMENT) RISK is the risk that changes in credit quality of a U.S. or foreign bank, insurance company or other financial institution or such entity’s failure to fulfill its obligations could cause the Portfolio’s investments to decline in value. Adverse developments in the banking or bond insurance industries also may negatively affect the Portfolio.

INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities.

INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio.

LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would.

LIQUIDITY FEE AND REDEMPTION GATE RISK is the risk that the Portfolio may impose a “liquidity fee” (up to 2%) or “redemption gate” that temporarily restricts your ability to sell shares for up to 10 business days if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors.

LIQUIDITY RISK is the risk that the Portfolio will not be able to pay redemption proceeds in a timely manner because of unusual market conditions, an unusually high volume of redemption requests, legal restrictions impairing its ability to sell particular securities at an advantageous market price or other reasons. Certain portfolio securities may be less liquid than others, which may make them difficult or impossible to sell at the time and the price that the Portfolio would like and the Portfolio may have to lower the price, sell other securities instead or forgo an investment opportunity. Any of these events could have a negative effect on the Portfolio’s performance.

MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results.

MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets.

PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities.

U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future.

You could lose money by investing in the Portfolio. Because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The Portfolio may impose a fee upon sale of your shares or may temporarily suspend your ability to sell shares if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.

<b>PORTFOLIO PERFORMANCE</b>

The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio from year to year. As of December 31, 2017 there were no Service Shares or Premier Shares outstanding for the Portfolio.

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

Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.

<b>CALENDAR YEAR TOTAL RETURN (SERVICE SHARES)*^†</b>

* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.87% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the first quarter of 2014.

^† The total return from January 1, 2017 through February 23, 2017 reflects the performance of the Portfolio’s Service Shares. As of February 24, 2017, there were no Service Shares outstanding. Performance information from that date through December 31, 2017 reflects that of the Portfolio’s Shares class. As of December 31, 2017 there were no Service Shares and no Premier Shares outstanding for the Portfolio. Service Shares and Premier Shares would have similar annual returns when compared with Shares because each class is invested in the same portfolio of securities. The annual return of the Shares class would differ from those of Service Shares and Premier Shares only to the extent that the classes do not have the same expenses.

<b>AVERAGE ANNUAL TOTAL RETURNS</b><br/>(For the periods ended December 31, 2017)

^[1]

Average Annual Total Returns	Inception Date	1-Year	5-Year	10-Year	Since Inception
Service and Premier Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO | Service Shares	Sep. 02, 2003	1.04%	0.34%	0.38%	1.22%

The 7-day yield for Shares of the Portfolio as of December 31, 2017: 1.41%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.

[1] “Other Expenses” have been estimated for the current fiscal year.

[2] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.15%. The “Total Annual Portfolio Operating Expenses After Expense Reimbursement” may be higher than the contractual limitation as a result of certain Portfolio expenses, including but not limited to service fees, that are not reimbursed. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

[1] The total return from January 1, 2017 through February 23, 2017 reflects the performance of the Portfolio’s Service Shares. As of February 24, 2017, there were no Service Shares outstanding. Performance information from that date through December 31, 2017 reflects that of the Portfolio’s Shares class. As of December 31, 2017 there were no Service Shares and no Premier Shares outstanding for the Portfolio. Service Shares and Premier Shares would have similar annual returns when compared with Shares because each class is invested in the same portfolio of securities. The annual return of the Shares class would differ from those of Service Shares and Premier Shares only to the extent that the classes do not have the same expenses.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Service and Premier Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>PRIME OBLIGATIONS PORTFOLIO—SERVICE SHARES AND PREMIER SHARES</b>
Objective [Heading]	rr_ObjectiveHeading	<b>INVESTMENT OBJECTIVE</b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.
Expense [Heading]	rr_ExpenseHeading	<b>FEES AND EXPENSES OF THE PORTFOLIO</b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Service Shares and Premier Shares of the Portfolio.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<b>Shareholder Fees </b>(fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other Expenses” have been estimated for the current fiscal year.
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>EXAMPLE</b>
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is intended to help you compare the cost of investing in Service Shares and Premier Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	<b>PRINCIPAL INVESTMENT STRATEGIES</b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Portfolio seeks to achieve its objective by investing in a broad range of high-quality, U.S. dollar-denominated government, bank and commercial obligations that are available in the money markets, including: Obligations of U.S. banks (including obligations of foreign branches of such banks); Obligations of foreign commercial banks; Commercial paper and other obligations issued or guaranteed by U.S. and foreign corporations and other issuers; Corporate bonds, notes, paper and other instruments that are of high quality; Asset-backed securities and asset-backed commercial paper; Securities issued or guaranteed as to principal and interest by the U.S. government or by its agencies, instrumentalities or sponsored enterprises and custodial receipts with respect thereto; Securities issued or guaranteed by one or more foreign governments or political subdivisions, agencies or instrumentalities; Repurchase agreements; and Municipal securities issued or guaranteed by state or local governmental bodies. Under normal market conditions, the Portfolio will invest at least 25% of its total assets in securities issued by companies in the financial services group of industries. Companies in the financial services group of industries include but are not limited to U.S. and non-U.S. companies involved in banking, mortgage, consumer or specialized finance, investment banking, securities brokerage, asset management and custody, insurance, financial investment, real estate and mortgage finance and financial conglomerates, and related asset-backed securities. The Portfolio may, however, for temporary defensive purposes, invest less than 25% of its total assets in the financial services industry if warranted due to adverse economic conditions or if investing less than 25% of its total assets in the financial services industry appears to be in the best interest of shareholders. The Portfolio operates as an “institutional money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As an “institutional money market fund” under Rule 2a-7, the Portfolio is (1) required to sell and redeem its shares at its net asset value (“NAV”) rounded to the fourth decimal place (e.g., $1.0000) reflecting market-based values of the Portfolio’s holdings, and (2) the Board of Trustees (the “Board”) will be permitted to impose a “liquidity fee” on redemptions from the Portfolio (up to 2%) or temporarily restrict redemptions from the Portfolio for up to 10 business days during a 90-day period, as described in more detail under “Purchase and Sales of Portfolio Shares.” The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.
Risk [Heading]	rr_RiskHeading	<b>PRINCIPAL RISKS</b>
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	ASSET-BACKED SECURITIES RISK. Asset-backed securities represent interests in pools of assets such as mortgages, automobile loans, credit card receivables and other financial assets. Asset-backed securities are subject to credit, interest rate, prepayment, extension, valuation and liquidity risk. These securities, in most cases, are not backed by the full faith and credit of the U.S. government and are subject to the risk of default on the underlying asset or loan, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain asset-backed securities. COMMERCIAL PAPER RISK. Investments in commercial paper are subject to the risk that the issuer cannot issue enough new commercial paper to satisfy its obligations with respect to its outstanding commercial paper, also known as rollover risk. Commercial paper is also susceptible to changes in the issuer’s financial condition or credit quality. In addition, under certain circumstances commercial paper may become illiquid or may suffer from reduced liquidity. Commercial paper is generally unsecured, which increases the credit risk associated with this type of investment. CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect. CURRENCY RISK is the risk that foreign currencies, securities that trade in or receive revenue in foreign currencies, or derivatives that provide exposure to foreign currencies will fluctuate in value relative to the U.S. dollar, adversely affecting the value of the Fund’s investments and its returns. Because the Portfolio’s NAV is determined on the basis of U.S. dollars, you may lose money if the local currency of a foreign market depreciates against the U.S. dollar, even if the market value of the Portfolio’s holdings appreciates. In addition, fluctuations in the exchange values of currencies could affect the economy or particular business operations of companies in a geographic region in which the Portfolio invests, causing an adverse impact on the Portfolio’s investments in the affected region. CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders. DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities. FINANCIAL SERVICES INDUSTRY RISK is the risk that, because the Portfolio will invest under normal market conditions at least 25% of its total assets in the financial services industry, the Portfolio will be subject to greater risk of loss by economic, business, political or other developments which generally affect this industry. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations. FLOATING NAV RISK is the risk that because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. FOREIGN SECURITIES RISK is the risk that a foreign security, even if it is a U.S. dollar-denominated foreign security, could lose value as a result of political, financial and economic events in foreign countries, more or less stringent foreign securities regulations and accounting and disclosure standards or other factors. In addition, the Portfolio will be subject to the risk that an issuer of foreign sovereign debt or the government authorities that control the repayment of the debt may be unable or unwilling to repay the principal or interest when due. GUARANTOR (OR CREDIT ENHANCEMENT) RISK is the risk that changes in credit quality of a U.S. or foreign bank, insurance company or other financial institution or such entity’s failure to fulfill its obligations could cause the Portfolio’s investments to decline in value. Adverse developments in the banking or bond insurance industries also may negatively affect the Portfolio. INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities. INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio. LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. LIQUIDITY FEE AND REDEMPTION GATE RISK is the risk that the Portfolio may impose a “liquidity fee” (up to 2%) or “redemption gate” that temporarily restricts your ability to sell shares for up to 10 business days if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors. LIQUIDITY RISK is the risk that the Portfolio will not be able to pay redemption proceeds in a timely manner because of unusual market conditions, an unusually high volume of redemption requests, legal restrictions impairing its ability to sell particular securities at an advantageous market price or other reasons. Certain portfolio securities may be less liquid than others, which may make them difficult or impossible to sell at the time and the price that the Portfolio would like and the Portfolio may have to lower the price, sell other securities instead or forgo an investment opportunity. Any of these events could have a negative effect on the Portfolio’s performance. MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results. MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of the securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets. PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities. U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future. You could lose money by investing in the Portfolio. Because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The Portfolio may impose a fee upon sale of your shares or may temporarily suspend your ability to sell shares if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.
Risk Lose Money [Text]	rr_RiskLoseMoney	<b>You could lose money by investing in the Portfolio.</b>
Risk Money Market Fund Price Fluctuates [Text]	rr_RiskMoneyMarketFundPriceFluctuates	<b>Because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</b>
Risk Money Market Fund May Impose Fees or Suspend Sales [Text]	rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales	<b>The Portfolio may impose a fee upon sale of your shares or may temporarily suspend your ability to sell shares if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors.</b>
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	<b>An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank.</b>
Risk Money Market Fund Sponsor May Not Provide Support [Text]	rr_RiskMoneyMarketFundSponsorMayNotProvideSupport	<b>The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.</b>
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>PORTFOLIO PERFORMANCE</b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio from year to year. As of December 31, 2017 there were no Service Shares or Premier Shares outstanding for the Portfolio. The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future. Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance of the Portfolio from year to year.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-637-1380
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	northerntrust.com/institutional
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	<b>CALENDAR YEAR TOTAL RETURN (SERVICE SHARES)*<sup>†</sup></b>
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	* Year to date total return for the nine months ended September 30, 2018 is [    ]%. For the periods shown in the bar chart above, the highest quarterly return was 0.87% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the first quarter of 2014. † The total return from January 1, 2017 through February 23, 2017 reflects the performance of the Portfolio’s Service Shares. As of February 24, 2017, there were no Service Shares outstanding. Performance information from that date through December 31, 2017 reflects that of the Portfolio’s Shares class. As of December 31, 2017 there were no Service Shares and no Premier Shares outstanding for the Portfolio. Service Shares and Premier Shares would have similar annual returns when compared with Shares because each class is invested in the same portfolio of securities. The annual return of the Shares class would differ from those of Service Shares and Premier Shares only to the extent that the classes do not have the same expenses.
Performance Table Heading	rr_PerformanceTableHeading	<b>AVERAGE ANNUAL TOTAL RETURNS</b><br/>(For the periods ended December 31, 2017)	[1]
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	The 7-day yield for Shares of the Portfolio as of December 31, 2017: 1.41%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.
Money Market Seven Day Yield, Caption [Text]	rr_MoneyMarketSevenDayYieldCaption	The 7-day yield for Shares of the Portfolio as of December 31, 2017
Money Market Seven Day Yield Phone	rr_MoneyMarketSevenDayYieldPhone	800-637-1380
Service and Premier Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO | Service Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.13%
Other Expenses	rr_OtherExpensesOverAssets	0.29%	[2]
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	0.25%
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.02%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.42%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.02%)	[3]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.40%
1 Year	rr_ExpenseExampleYear01	$ 41
3 Years	rr_ExpenseExampleYear03	133
5 Years	rr_ExpenseExampleYear05	233
10 Years	rr_ExpenseExampleYear10	$ 528
2008	rr_AnnualReturn2008	1.97%
2009	rr_AnnualReturn2009	0.17%
2010	rr_AnnualReturn2010	0.02%
2011	rr_AnnualReturn2011	0.02%
2012	rr_AnnualReturn2012	0.07%
2013	rr_AnnualReturn2013	0.05%
2014	rr_AnnualReturn2014	0.02%
2015	rr_AnnualReturn2015	0.07%
2016	rr_AnnualReturn2016	0.49%
2017	rr_AnnualReturn2017	1.04%
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to date total return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest quarterly return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.87%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest quarterly return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2014
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	none
1-Year	rr_AverageAnnualReturnYear01	1.04%
5-Year	rr_AverageAnnualReturnYear05	0.34%
10-Year	rr_AverageAnnualReturnYear10	0.38%
Since Inception	rr_AverageAnnualReturnSinceInception	1.22%
Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 02, 2003
Money Market Seven Day Yield	rr_MoneyMarketSevenDayYield	1.41%
Service and Premier Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO | Premier Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.13%
Other Expenses	rr_OtherExpensesOverAssets	0.54%	[2]
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	0.50%
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.02%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.67%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.02%)	[3]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.65%
1 Year	rr_ExpenseExampleYear01	$ 66
3 Years	rr_ExpenseExampleYear03	212
5 Years	rr_ExpenseExampleYear05	371
10 Years	rr_ExpenseExampleYear10	$ 833

[1] The total return from January 1, 2017 through February 23, 2017 reflects the performance of the Portfolio’s Service Shares. As of February 24, 2017, there were no Service Shares outstanding. Performance information from that date through December 31, 2017 reflects that of the Portfolio’s Shares class. As of December 31, 2017 there were no Service Shares and no Premier Shares outstanding for the Portfolio. Service Shares and Premier Shares would have similar annual returns when compared with Shares because each class is invested in the same portfolio of securities. The annual return of the Shares class would differ from those of Service Shares and Premier Shares only to the extent that the classes do not have the same expenses.

[2] “Other Expenses” have been estimated for the current fiscal year.

[3] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.15%. The “Total Annual Portfolio Operating Expenses After Expense Reimbursement” may be higher than the contractual limitation as a result of certain Portfolio expenses, including but not limited to service fees, that are not reimbursed. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

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Williams Capital Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO

<b>PRIME OBLIGATIONS PORTFOLIO</b>

<b>INVESTMENT OBJECTIVE</b>

The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.

<b>FEES AND EXPENSES OF THE PORTFOLIO</b>

This table describes the fees and expenses that you may pay if you buy and hold Williams Capital Shares of the Portfolio.

<b>Shareholder Fees </b>(fees paid directly from your investment)

Shareholder Fees	Williams Capital Shares NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO Williams Capital Shares USD ($)
Shareholder Fees (fees paid directly from your investment)	none

<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)

Annual Fund Operating Expenses	Williams Capital Shares NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO Williams Capital Shares
Management Fees	0.13%
Other Expenses	0.04%	[1]
Transfer Agent Fees	0.02%
Service Fees	none
Other Operating Expenses	0.02%
Total Annual Portfolio Operating Expenses	0.17%
Expense Reimbursement	(0.02%)	[2]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	0.15%

<b>EXAMPLE</b>

The following Example is intended to help you compare the cost of investing in Williams Capital Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example	1 Year	3 Years	5 Years	10 Years
Williams Capital Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO | Williams Capital Shares | USD ($)	15	53	94	215

<b>PRINCIPAL INVESTMENT STRATEGIES</b>

The Portfolio seeks to achieve its objective by investing in a broad range of high-quality, U.S. dollar-denominated government, bank and commercial obligations that are available in the money markets, including:

• Obligations of U.S. banks (including obligations of foreign branches of such banks);

• Obligations of foreign commercial banks;

• Commercial paper and other obligations issued or guaranteed by U.S. and foreign corporations and other issuers;

• Corporate bonds, notes, paper and other instruments that are of high quality;

• Asset-backed securities and asset-backed commercial paper;

• Securities issued or guaranteed as to principal and interest by the U.S. government or by its agencies, instrumentalities or sponsored enterprises and custodial receipts with respect thereto;

• Securities issued or guaranteed by one or more foreign governments or political subdivisions, agencies or instrumentalities;

• Repurchase agreements; and

• Municipal securities issued or guaranteed by state or local governmental bodies.

Under normal market conditions, the Portfolio will invest at least 25% of its total assets in securities issued by companies in the financial services group of industries. Companies in the financial services group of industries include but are not limited to U.S. and non-U.S. companies involved in banking, mortgage, consumer or specialized finance, investment banking, securities brokerage, asset management and custody, insurance, financial investment, real estate and mortgage finance and financial conglomerates, and related asset-backed securities. The Portfolio may, however, for temporary defensive purposes, invest less than 25% of its total assets in the financial services industry if warranted due to adverse economic conditions or if investing less than 25% of its total assets in the financial services industry appears to be in the best interest of shareholders.

The Portfolio operates as an “institutional money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As an “institutional money market fund” under Rule 2a-7, the Portfolio is (1) required to sell and redeem its shares at its net asset value (“NAV”) rounded to the fourth decimal place (e.g., $1.0000) reflecting market-based values of the Portfolio’s holdings, and (2) the Board of Trustees (the “Board”) will be permitted to impose a “liquidity fee” on redemptions from the Portfolio (up to 2%) or temporarily restrict redemptions from the Portfolio for up to 10 business days during a 90-day period, as described in more detail under “Purchase and Sales of Portfolio Shares.”

The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.

<b>PRINCIPAL RISKS</b>

ASSET-BACKED SECURITIES RISK. Asset-backed securities represent interests in pools of assets such as mortgages, automobile loans, credit card receivables and other financial assets. Asset-backed securities are subject to credit, interest rate, prepayment, extension, valuation and liquidity risk. These securities, in most cases, are not backed by the full faith and credit of the U.S. government and are subject to the risk of default on the underlying asset or loan, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain asset-backed securities.

COMMERCIAL PAPER RISK. Investments in commercial paper are subject to the risk that the issuer cannot issue enough new commercial paper to satisfy its obligations with respect to its outstanding commercial paper, also known as rollover risk. Commercial paper is also susceptible to changes in the issuer’s financial condition or credit quality. In addition, under certain circumstances commercial paper may become illiquid or may suffer from reduced liquidity. Commercial paper is generally unsecured, which increases the credit risk associated with this type of investment.

CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect.

CURRENCY RISK is the risk that foreign currencies, securities that trade in or receive revenue in foreign currencies, or derivatives that provide exposure to foreign currencies will fluctuate in value relative to the U.S. dollar, adversely affecting the value of the Fund’s investments and its returns. Because the Portfolio’s NAV is determined on the basis of U.S. dollars, you may lose money if the local currency of a foreign market depreciates against the U.S. dollar, even if the market value of the Portfolio’s holdings appreciates. In addition, fluctuations in the exchange values of currencies could affect the economy or particular business operations of companies in a geographic region in which the Portfolio invests, causing an adverse impact on the Portfolio’s investments in the affected region.

CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders.

DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities.

FINANCIAL SERVICES INDUSTRY RISK is the risk that, because the Portfolio will invest under normal market conditions at least 25% of its total assets in the financial services industry, the Portfolio will be subject to greater risk of loss by economic, business, political or other developments which generally affect this industry. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

FLOATING NAV RISK is the risk that because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.

FOREIGN SECURITIES RISK is the risk that a foreign security, even if it is a U.S. dollar-denominated foreign security, could lose value as a result of political, financial and economic events in foreign countries, more or less stringent foreign securities regulations and accounting and disclosure standards or other factors. In addition, the Portfolio will be subject to the risk that an issuer of foreign sovereign debt or the government authorities that control the repayment of the debt may be unable or unwilling to repay the principal or interest when due.

GUARANTOR (OR CREDIT ENHANCEMENT) RISK is the risk that changes in credit quality of a U.S. or foreign bank, insurance company or other financial institution or such entity’s failure to fulfill its obligations could cause the Portfolio’s investments to decline in value. Adverse developments in the banking or bond insurance industries also may negatively affect the Portfolio.

INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities.

INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio.

LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would.

LIQUIDITY FEE AND REDEMPTION GATE RISK is the risk that the Portfolio may impose a “liquidity fee” (up to 2%) or “redemption gate” that temporarily restricts your ability to sell shares for up to 10 business days if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors.

LIQUIDITY RISK is the risk that the Portfolio will not be able to pay redemption proceeds in a timely manner because of unusual market conditions, an unusually high volume of redemption requests, legal restrictions impairing its ability to sell particular securities at an advantageous market price or other reasons. Certain portfolio securities may be less liquid than others, which may make them difficult or impossible to sell at the time and the price that the Portfolio would like and the Portfolio may have to lower the price, sell other securities instead or forgo an investment opportunity. Any of these events could have a negative effect on the Portfolio’s performance.

MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results.

MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets.

PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities.

U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future.

You could lose money by investing in the Portfolio. Because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The Portfolio may impose a fee upon sale of your shares or may temporarily suspend your ability to sell shares if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.

<b>PORTFOLIO PERFORMANCE</b>

Williams Capital Shares of the Portfolio had not commenced operations as of December 31, 2017 and, therefore, no performance information for Williams Capital Shares is included in this Prospectus. The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance from year to year of Shares, another class of shares of the Portfolio that is offered in a separate prospectus.

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

Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.

<b>CALENDAR YEAR TOTAL RETURN (SHARES)*^†</b>

* Year to date total return for the nine months ended September 30, 2018 is [    ] %. For the periods shown in the bar chart above, the highest quarterly return was 0.87% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the first quarter of 2014.

^† The returns in the bar chart and table above are for Shares, another class of shares of the Portfolio that is offered in a separate prospectus. Williams Capital Shares would have substantially similar annual returns because Williams Capital Shares are invested in the same portfolio of securities and the annual returns would differ only to the extent that the classes have different expenses.

<b>AVERAGE ANNUAL TOTAL RETURNS</b><br/>(For the periods ended December 31, 2017)

^[1]

Average Annual Total Returns	Inception Date	1-Year	5-Year	10-Year	Since Inception
Williams Capital Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO | Shares	Aug. 21, 2003	1.04%	0.34%	0.46%	1.34%

The 7-day yield for Shares of the Portfolio as of December 31, 2017: 1.41%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.

[1] “Other Expenses” have been estimated for the current fiscal year.

[2] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.15%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

[1] The returns in the bar chart and table above are for Shares, another class of shares of the Portfolio that is offered in a separate prospectus. Williams Capital Shares would have substantially similar annual returns because Williams Capital Shares are invested in the same portfolio of securities and the annual returns would differ only to the extent that the classes have different expenses.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Williams Capital Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>PRIME OBLIGATIONS PORTFOLIO</b>
Objective [Heading]	rr_ObjectiveHeading	<b>INVESTMENT OBJECTIVE</b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.
Expense [Heading]	rr_ExpenseHeading	<b>FEES AND EXPENSES OF THE PORTFOLIO</b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Williams Capital Shares of the Portfolio.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<b>Shareholder Fees </b>(fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	“Other Expenses” have been estimated for the current fiscal year.
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>EXAMPLE</b>
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is intended to help you compare the cost of investing in Williams Capital Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	<b>PRINCIPAL INVESTMENT STRATEGIES</b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Portfolio seeks to achieve its objective by investing in a broad range of high-quality, U.S. dollar-denominated government, bank and commercial obligations that are available in the money markets, including: Obligations of U.S. banks (including obligations of foreign branches of such banks); Obligations of foreign commercial banks; Commercial paper and other obligations issued or guaranteed by U.S. and foreign corporations and other issuers; Corporate bonds, notes, paper and other instruments that are of high quality; Asset-backed securities and asset-backed commercial paper; Securities issued or guaranteed as to principal and interest by the U.S. government or by its agencies, instrumentalities or sponsored enterprises and custodial receipts with respect thereto; Securities issued or guaranteed by one or more foreign governments or political subdivisions, agencies or instrumentalities; Repurchase agreements; and Municipal securities issued or guaranteed by state or local governmental bodies. Under normal market conditions, the Portfolio will invest at least 25% of its total assets in securities issued by companies in the financial services group of industries. Companies in the financial services group of industries include but are not limited to U.S. and non-U.S. companies involved in banking, mortgage, consumer or specialized finance, investment banking, securities brokerage, asset management and custody, insurance, financial investment, real estate and mortgage finance and financial conglomerates, and related asset-backed securities. The Portfolio may, however, for temporary defensive purposes, invest less than 25% of its total assets in the financial services industry if warranted due to adverse economic conditions or if investing less than 25% of its total assets in the financial services industry appears to be in the best interest of shareholders. The Portfolio operates as an “institutional money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As an “institutional money market fund” under Rule 2a-7, the Portfolio is (1) required to sell and redeem its shares at its net asset value (“NAV”) rounded to the fourth decimal place (e.g., $1.0000) reflecting market-based values of the Portfolio’s holdings, and (2) the Board of Trustees (the “Board”) will be permitted to impose a “liquidity fee” on redemptions from the Portfolio (up to 2%) or temporarily restrict redemptions from the Portfolio for up to 10 business days during a 90-day period, as described in more detail under “Purchase and Sales of Portfolio Shares.” The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.
Risk [Heading]	rr_RiskHeading	<b>PRINCIPAL RISKS</b>
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	ASSET-BACKED SECURITIES RISK. Asset-backed securities represent interests in pools of assets such as mortgages, automobile loans, credit card receivables and other financial assets. Asset-backed securities are subject to credit, interest rate, prepayment, extension, valuation and liquidity risk. These securities, in most cases, are not backed by the full faith and credit of the U.S. government and are subject to the risk of default on the underlying asset or loan, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain asset-backed securities. COMMERCIAL PAPER RISK. Investments in commercial paper are subject to the risk that the issuer cannot issue enough new commercial paper to satisfy its obligations with respect to its outstanding commercial paper, also known as rollover risk. Commercial paper is also susceptible to changes in the issuer’s financial condition or credit quality. In addition, under certain circumstances commercial paper may become illiquid or may suffer from reduced liquidity. Commercial paper is generally unsecured, which increases the credit risk associated with this type of investment. CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect. CURRENCY RISK is the risk that foreign currencies, securities that trade in or receive revenue in foreign currencies, or derivatives that provide exposure to foreign currencies will fluctuate in value relative to the U.S. dollar, adversely affecting the value of the Fund’s investments and its returns. Because the Portfolio’s NAV is determined on the basis of U.S. dollars, you may lose money if the local currency of a foreign market depreciates against the U.S. dollar, even if the market value of the Portfolio’s holdings appreciates. In addition, fluctuations in the exchange values of currencies could affect the economy or particular business operations of companies in a geographic region in which the Portfolio invests, causing an adverse impact on the Portfolio’s investments in the affected region. CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders. DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities. FINANCIAL SERVICES INDUSTRY RISK is the risk that, because the Portfolio will invest under normal market conditions at least 25% of its total assets in the financial services industry, the Portfolio will be subject to greater risk of loss by economic, business, political or other developments which generally affect this industry. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations. FLOATING NAV RISK is the risk that because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. FOREIGN SECURITIES RISK is the risk that a foreign security, even if it is a U.S. dollar-denominated foreign security, could lose value as a result of political, financial and economic events in foreign countries, more or less stringent foreign securities regulations and accounting and disclosure standards or other factors. In addition, the Portfolio will be subject to the risk that an issuer of foreign sovereign debt or the government authorities that control the repayment of the debt may be unable or unwilling to repay the principal or interest when due. GUARANTOR (OR CREDIT ENHANCEMENT) RISK is the risk that changes in credit quality of a U.S. or foreign bank, insurance company or other financial institution or such entity’s failure to fulfill its obligations could cause the Portfolio’s investments to decline in value. Adverse developments in the banking or bond insurance industries also may negatively affect the Portfolio. INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities. INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio. LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. LIQUIDITY FEE AND REDEMPTION GATE RISK is the risk that the Portfolio may impose a “liquidity fee” (up to 2%) or “redemption gate” that temporarily restricts your ability to sell shares for up to 10 business days if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors. LIQUIDITY RISK is the risk that the Portfolio will not be able to pay redemption proceeds in a timely manner because of unusual market conditions, an unusually high volume of redemption requests, legal restrictions impairing its ability to sell particular securities at an advantageous market price or other reasons. Certain portfolio securities may be less liquid than others, which may make them difficult or impossible to sell at the time and the price that the Portfolio would like and the Portfolio may have to lower the price, sell other securities instead or forgo an investment opportunity. Any of these events could have a negative effect on the Portfolio’s performance. MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results. MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets. PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities. U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future. You could lose money by investing in the Portfolio. Because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The Portfolio may impose a fee upon sale of your shares or may temporarily suspend your ability to sell shares if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.
Risk Lose Money [Text]	rr_RiskLoseMoney	<b>You could lose money by investing in the Portfolio.</b>
Risk Money Market Fund Price Fluctuates [Text]	rr_RiskMoneyMarketFundPriceFluctuates	<b>Because the share price of the Portfolio will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</b>
Risk Money Market Fund May Impose Fees or Suspend Sales [Text]	rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales	<b>The Portfolio may impose a fee upon sale of your shares or may temporarily suspend your ability to sell shares if the Portfolio’s liquidity falls below required minimums because of market conditions or other factors.</b>
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	<b>An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank.</b>
Risk Money Market Fund Sponsor May Not Provide Support [Text]	rr_RiskMoneyMarketFundSponsorMayNotProvideSupport	<b>The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.</b>
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>PORTFOLIO PERFORMANCE</b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	Williams Capital Shares of the Portfolio had not commenced operations as of December 31, 2017 and, therefore, no performance information for Williams Capital Shares is included in this Prospectus. The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance from year to year of Shares, another class of shares of the Portfolio that is offered in a separate prospectus. The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future. Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing changes in the performance from year to year of Shares, another class of shares of the Portfolio that is offered in a separate prospectus.
Performance One Year or Less [Text]	rr_PerformanceOneYearOrLess	Williams Capital Shares of the Portfolio had not commenced operations as of December 31, 2017 and, therefore, no performance information for Williams Capital Shares is included in this Prospectus.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-637-1380
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	northerntrust.com/institutional
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	<b>CALENDAR YEAR TOTAL RETURN (SHARES)*<sup>†</sup></b>
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	* Year to date total return for the nine months ended September 30, 2018 is [    ] %. For the periods shown in the bar chart above, the highest quarterly return was 0.87% in the first quarter of 2008, and the lowest quarterly return was 0.00% in the first quarter of 2014. † The returns in the bar chart and table above are for Shares, another class of shares of the Portfolio that is offered in a separate prospectus. Williams Capital Shares would have substantially similar annual returns because Williams Capital Shares are invested in the same portfolio of securities and the annual returns would differ only to the extent that the classes have different expenses.
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	The returns in the bar chart and table above are for Shares, another class of shares of the Portfolio that is offered in a separate prospectus. Williams Capital Shares would have substantially similar annual returns because Williams Capital Shares are invested in the same portfolio of securities and the annual returns would differ only to the extent that the classes have different expenses.
Performance Table Heading	rr_PerformanceTableHeading	<b>AVERAGE ANNUAL TOTAL RETURNS</b><br/>(For the periods ended December 31, 2017)	[1]
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	The 7-day yield for Shares of the Portfolio as of December 31, 2017: 1.41%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.
Money Market Seven Day Yield, Caption [Text]	rr_MoneyMarketSevenDayYieldCaption	The 7-day yield for Shares of the Portfolio as of December 31, 2017
Money Market Seven Day Yield Phone	rr_MoneyMarketSevenDayYieldPhone	800-637-1380
Williams Capital Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO | Williams Capital Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.13%
Other Expenses	rr_OtherExpensesOverAssets	0.04%	[2]
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	none
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.02%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.17%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.02%)	[3]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.15%
1 Year	rr_ExpenseExampleYear01	$ 15
3 Years	rr_ExpenseExampleYear03	53
5 Years	rr_ExpenseExampleYear05	94
10 Years	rr_ExpenseExampleYear10	$ 215
Williams Capital Shares | NORTHERN INSTITUTIONAL PRIME OBLIGATIONS PORTFOLIO | Shares
Risk/Return:	rr_RiskReturnAbstract
2008	rr_AnnualReturn2008	2.23%
2009	rr_AnnualReturn2009	0.39%
2010	rr_AnnualReturn2010	0.15%
2011	rr_AnnualReturn2011	0.08%
2012	rr_AnnualReturn2012	0.08%
2013	rr_AnnualReturn2013	0.05%
2014	rr_AnnualReturn2014	0.02%
2015	rr_AnnualReturn2015	0.07%
2016	rr_AnnualReturn2016	0.49%
2017	rr_AnnualReturn2017	1.04%
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to date total return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest quarterly return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.87%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest quarterly return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2014
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	none
1-Year	rr_AverageAnnualReturnYear01	1.04%
5-Year	rr_AverageAnnualReturnYear05	0.34%
10-Year	rr_AverageAnnualReturnYear10	0.46%
Since Inception	rr_AverageAnnualReturnSinceInception	1.34%
Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 21, 2003
Money Market Seven Day Yield	rr_MoneyMarketSevenDayYield	1.41%

[1] The returns in the bar chart and table above are for Shares, another class of shares of the Portfolio that is offered in a separate prospectus. Williams Capital Shares would have substantially similar annual returns because Williams Capital Shares are invested in the same portfolio of securities and the annual returns would differ only to the extent that the classes have different expenses.

[2] “Other Expenses” have been estimated for the current fiscal year.

[3] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.15%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

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Williams Capital Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO

<b>U.S. GOVERNMENT SELECT PORTFOLIO</b>

<b>INVESTMENT OBJECTIVE </b>

The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.

<b>FEES AND EXPENSES OF THE PORTFOLIO </b>

This table describes the fees and expenses that you may pay if you buy and hold Williams Capital Shares of the Portfolio.

<b>Shareholder Fees </b> (fees paid directly from your investment)

Shareholder Fees	Williams Capital Shares NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO Williams Capital Shares USD ($)
Shareholder Fees (fees paid directly from your investment)	none

<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)

Annual Fund Operating Expenses	Williams Capital Shares NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO Williams Capital Shares
Management Fees	0.18%
Other Expenses	0.03%
Transfer Agent Fees	0.02%
Service Fees	none
Other Operating Expenses	0.01%
Total Annual Portfolio Operating Expenses	0.21%
Expense Reimbursement	(0.01%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	0.20%

<b>EXAMPLE </b>

The following Example is intended to help you compare the cost of investing in Williams Capital Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example	1 Year	3 Years	5 Years	10 Years
Williams Capital Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO | Williams Capital Shares | USD ($)	20	67	117	267

<b>PRINCIPAL INVESTMENT STRATEGIES </b>

The Portfolio seeks to achieve its objective by investing, under normal circumstances, substantially all (and at least 99.5%) of its total assets in cash, securities issued or guaranteed as to principal and interest by the U.S. government or by a person controlled or supervised by and acting as an instrumentality of the U.S. government pursuant to authority granted by the Congress of the United States or any certificate of deposit of any of the foregoing, and repurchase agreements that are fully collateralized by cash or such securities. Subject to the foregoing 99.5% investment strategy requirement, under normal circumstances, the Portfolio will seek to acquire only those U.S. government securities paying interest that generally is exempt from state income taxation. The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in U.S. government securities and repurchase agreements collateralized solely by U.S. government securities.

The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur.

The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.

<b>PRINCIPAL RISKS </b>

CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect.

CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders.

DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities.

INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities.

INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio.

LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would.

MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results.

MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets.

PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities.

STABLE NAV RISK is the risk that the Portfolio will not be able to maintain an NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV.

U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future.

You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.

<b>PORTFOLIO PERFORMANCE</b>

The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing the performance of the Portfolio for a one year period and since inception.

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

Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.

<b>CALENDAR YEAR TOTAL RETURN (WILLIAMS CAPITAL SHARES)*</b>

* Year to date total return for the nine months ended September 30, 2018 is [    ] %. For the periods shown in the bar chart above, the highest quarterly return was 0.24% in the fourth quarter of 2017, and the lowest quarterly return was 0.00% in the third quarter of 2015.

<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)

Average Annual Total Returns	Inception Date	1-Year	Since Inception
Williams Capital Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO | Williams Capital Shares	Sep. 15, 2014	0.73%	0.29%

The 7-day yield for Williams Capital Shares of the Portfolio as of December 31, 2017: 1.13%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.

[1] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.20%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Williams Capital Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>U.S. GOVERNMENT SELECT PORTFOLIO</b>
Objective [Heading]	rr_ObjectiveHeading	<b>INVESTMENT OBJECTIVE </b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Portfolio seeks to maximize current income to the extent consistent with the preservation of capital and maintenance of liquidity by investing exclusively in high quality money market instruments.
Expense [Heading]	rr_ExpenseHeading	<b>FEES AND EXPENSES OF THE PORTFOLIO </b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Williams Capital Shares of the Portfolio.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<b>Shareholder Fees </b> (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Portfolio Operating Expenses </b>(expenses that you pay each year as<br/>a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>EXAMPLE </b>
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is intended to help you compare the cost of investing in Williams Capital Shares of the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	<b>PRINCIPAL INVESTMENT STRATEGIES </b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Portfolio seeks to achieve its objective by investing, under normal circumstances, substantially all (and at least 99.5%) of its total assets in cash, securities issued or guaranteed as to principal and interest by the U.S. government or by a person controlled or supervised by and acting as an instrumentality of the U.S. government pursuant to authority granted by the Congress of the United States or any certificate of deposit of any of the foregoing, and repurchase agreements that are fully collateralized by cash or such securities. Subject to the foregoing 99.5% investment strategy requirement, under normal circumstances, the Portfolio will seek to acquire only those U.S. government securities paying interest that generally is exempt from state income taxation. The Portfolio, under normal circumstances, will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in U.S. government securities and repurchase agreements collateralized solely by U.S. government securities. The Portfolio operates as a “government money market fund” under Rule 2a-7 of the Investment Company Act of 1940, as amended. As a “government money market fund” under Rule 2a-7, the Portfolio (1) is permitted to use the amortized cost method of valuation to seek to maintain a stable net asset value (“NAV”) of $1.00 share price, and (2) is not required to impose a liquidity fee and/or a redemption gate on fund redemptions that might apply to other types of money market funds should certain triggering events specified in Rule 2a-7 occur. The Securities and Exchange Commission (“SEC”) imposes strict requirements on the investment quality, maturity, diversification and liquidity of the Portfolio’s investments. Accordingly, the Portfolio’s investments must have a remaining maturity of no more than 397 days and must be high quality. The Portfolio’s investment adviser may consider, among other things, credit, interest rate and prepayment risks as well as general market conditions when deciding whether to buy or sell investments for the Portfolio.
Risk [Heading]	rr_RiskHeading	<b>PRINCIPAL RISKS </b>
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Portfolio’s investments and its yield. Changes in the credit rating of a debt security held by the Portfolio could have a similar effect. CYBERSECURITY RISK is the risk of an unauthorized breach and access to fund assets, customer data (including private shareholder information), or proprietary information, or the risk of an incident occurring that causes the Portfolio, the investment adviser, custodian, transfer agent, distributor and other service providers and financial intermediaries to suffer data breaches, data corruption or lose operational functionality. Successful cyber-attacks or other cyber-failures or events affecting the Portfolio or its service providers may adversely impact the Portfolio or its shareholders. DEBT EXTENSION RISK is the risk that an issuer will exercise its right to pay principal on an obligation held by the Portfolio (such as an asset-backed security) later than expected. This may happen during a period of rising interest rates. Under these circumstances, the value of the obligation will decrease and the Portfolio will suffer from the inability to invest in higher yielding securities. INCOME RISK is the risk that falling interest rates will cause the Portfolio’s income to decline. Income risk is generally higher for short-term debt securities. INTEREST RATE RISK is the risk that during periods of rising interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be lower than prevailing market rates; in periods of falling interest rates, the Portfolio’s yield (and the market value of its securities) will tend to be higher. If interest rates rise, the Portfolio’s yield may not increase proportionately. The risks associated with increasing interest rates are heightened given that interest rates are near historic lows, but are expected to increase in the future with unpredictable effects on the markets and the Portfolio’s investments. A low interest rate environment may prevent the Portfolio from providing a positive yield or paying Portfolio expenses out of Portfolio assets and could lead to a decline in the Portfolio’s share price. Fluctuations in interest rates may also affect the liquidity of fixed income securities and instruments held by the Portfolio. LARGE SHAREHOLDER TRANSACTIONS RISK is the risk that the Portfolio may experience adverse effects when certain large shareholders purchase or redeem large amounts of shares of the Portfolio. Such large shareholder redemptions may cause the Portfolio to sell its securities at times it would not otherwise do so, which may negatively impact its liquidity. In addition, large redemptions could lead to an increase in the Portfolio’s expense ratio due to a smaller asset base. Large Portfolio share purchases may adversely affect the Portfolio’s performance to the extent that the Portfolio is delayed in investing new cash and is required to maintain a larger cash position than it ordinarily would. MANAGEMENT RISK is the risk that a strategy used by the Portfolio’s investment adviser may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by the investment adviser may cause unintended results. MARKET RISK is the risk that general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment could cause the value of your investment in the Portfolio, or its yield, to decline. The market value of securities in which the Portfolio invests may go up or down in response to the prospects of particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets. PREPAYMENT (OR CALL) RISK is the risk that prepayment of the underlying mortgages or other collateral of some fixed-income securities may result in a decreased rate of return and a decline in value of those securities. STABLE NAV RISK is the risk that the Portfolio will not be able to maintain an NAV per share of $1.00 at all times. A significant enough market disruption or drop in market prices of securities held by the Portfolio, especially at a time when the Portfolio needs to sell securities to meet shareholder redemption requests, could cause the value of the Portfolio’s shares to decrease to a price less than $1.00 per share. If the Portfolio fails to maintain a stable NAV (or if there is a perceived threat of such a failure), the Portfolio could be subject to increased redemption activity, which could adversely affect its NAV. U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Portfolio are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future. You could lose money by investing in the Portfolio. Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time.
Risk Lose Money [Text]	rr_RiskLoseMoney	<b>You could lose money by investing in the Portfolio. </b>
Risk Money Market Fund May Not Preserve Dollar [Text]	rr_RiskMoneyMarketFundMayNotPreserveDollar	<b>Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, the Portfolio cannot guarantee it will do so. </b>
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	<b> An investment in the Portfolio is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. </b>
Risk Money Market Fund Sponsor May Not Provide Support [Text]	rr_RiskMoneyMarketFundSponsorMayNotProvideSupport	<b>The Portfolio’s sponsor has no legal obligation to provide financial support to the Portfolio, and you should not expect that the sponsor will provide financial support to the Portfolio at any time. </b>
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>PORTFOLIO PERFORMANCE</b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing the performance of the Portfolio for a one year period and since inception. The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future. Updated performance information for the Portfolio is available and may be obtained on the Portfolio’s website at northerntrust.com/institutional or by calling 800-637-1380.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart and table that follow provide an indication of the risks of investing in the Portfolio by showing the performance of the Portfolio for a one year period and since inception.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-637-1380
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	northerntrust.com/institutional
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Portfolio’s past performance is not necessarily an indication of how the Portfolio will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	<b>CALENDAR YEAR TOTAL RETURN (WILLIAMS CAPITAL SHARES)*</b>
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	* Year to date total return for the nine months ended September 30, 2018 is [    ] %. For the periods shown in the bar chart above, the highest quarterly return was 0.24% in the fourth quarter of 2017, and the lowest quarterly return was 0.00% in the third quarter of 2015.
Performance Table Heading	rr_PerformanceTableHeading	<b>AVERAGE ANNUAL TOTAL RETURNS </b><br/>(For the periods ended December 31, 2017)
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	The 7-day yield for Williams Capital Shares of the Portfolio as of December 31, 2017: 1.13%. For the current 7-day yield call 800-637-1380 or visit northerntrust.com/institutional.
Money Market Seven Day Yield, Caption [Text]	rr_MoneyMarketSevenDayYieldCaption	The 7-day yield for Williams Capital Shares of the Portfolio as of December 31, 2017
Money Market Seven Day Yield Phone	rr_MoneyMarketSevenDayYieldPhone	800-637-1380
Williams Capital Shares | NORTHERN INSTITUTIONAL U.S. GOVERNMENT SELECT PORTFOLIO | Williams Capital Shares
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.18%
Other Expenses	rr_OtherExpensesOverAssets	0.03%
Transfer Agent Fees	rr_Component1OtherExpensesOverAssets	0.02%
Service Fees	rr_Component2OtherExpensesOverAssets	none
Other Operating Expenses	rr_Component3OtherExpensesOverAssets	0.01%
Total Annual Portfolio Operating Expenses	rr_ExpensesOverAssets	0.21%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.01%)	[1]
Total Annual Portfolio Operating Expenses After Expense Reimbursement	rr_NetExpensesOverAssets	0.20%
1 Year	rr_ExpenseExampleYear01	$ 20
3 Years	rr_ExpenseExampleYear03	67
5 Years	rr_ExpenseExampleYear05	117
10 Years	rr_ExpenseExampleYear10	$ 267
2015	rr_AnnualReturn2015	0.02%
2016	rr_AnnualReturn2016	0.23%
2017	rr_AnnualReturn2017	0.73%
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to date total return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest quarterly return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2017
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.24%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest quarterly return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2015
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	none
1-Year	rr_AverageAnnualReturnYear01	0.73%
Since Inception	rr_AverageAnnualReturnSinceInception	0.29%
Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 15, 2014
Money Market Seven Day Yield	rr_MoneyMarketSevenDayYield	1.13%

[1] Northern Trust Investments, Inc. has contractually agreed to reimburse a portion of the operating expenses of the Portfolio (other than certain excepted expenses, i.e., acquired fund fees and expenses, service fees, the compensation paid to each Independent Trustee of the Trust, expenses of third party consultants engaged by the Board of Trustees, membership dues paid to the Investment Company Institute and Mutual Fund Directors Forum, expenses in connection with the negotiation and renewal of the revolving credit facility, extraordinary expenses and interest) to the extent the “Total Annual Portfolio Operating Expenses” exceed 0.20%. This contractual limitation will remain in effect for at least one year from the effective date of this Prospectus and may not be terminated before that time without the approval of the Board of Trustees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	NORTHERN INSTITUTIONAL FUNDS
Prospectus Date	rr_ProspectusDate	Nov. 13, 2018
Document Creation Date	dei_DocumentCreationDate	Sep. 14, 2018