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Label Element Value
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading ACTIVE ASSETS TAX-FREE TRUST
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

Active Assets Tax-Free Trust is a money market fund that seeks to provide as high a level of daily income exempt from federal personal income tax as is consistent with stability of principal and liquidity.

Expense [Heading] rr_ExpenseHeading Fees and Expenses
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock

The table below describes the fees and expenses that you may pay if you buy and hold shares of the Fund. The Fund is a no-load fund. The Fund does not impose any sales charges.

Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock

The example below is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds.


The example assumes that you invest $10,000 in the Fund, your investment has a 5% return each year, and the Fund'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 Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock

The Fund will invest in high quality, short-term securities that are normally municipal obligations that pay interest exempt from federal income taxes. The Adviser seeks to maintain the Fund's share price at $1.00. The Fund has a fundamental policy of investing at least 80% of its net assets in securities the interest on which is exempt from federal personal income tax. This policy may not be changed without shareholder approval. In addition, the Fund may invest up to 20% of its net assets in securities that pay interest income subject to the "alternative minimum tax," and some taxpayers may have to pay tax on a Fund distribution of this income. For more information, please refer to the section of this Prospectus entitled "Tax Consequences."


Municipal obligations are securities issued by state and local governments and their agencies. These securities typically are "general obligation" or "revenue" bonds, notes or commercial paper, including participations in lease obligations and installment purchase contracts of municipalities.


The Fund may purchase debt obligations that have fixed, variable or floating rates of interest. The interest rates payable on variable or floating rate obligations may fluctuate based upon changes in market rates.


The Fund's investments may include tender option bonds and custodial receipts. Tender option bonds are municipal obligations held pursuant to a custodial arrangement and issued pursuant to an agreement with a third-party such as a bank or financial institution to provide the holder with the option of tendering the bond at periodic intervals. The holder of the tender option bond effectively holds a demand obligation that bears interest at prevailing short-term rates. Custodial receipts represent interests in future interest and/or principal payments on U.S. government securities or municipal obligations. Additionally, the Fund may invest in investment companies, including money market funds some or all of which may be advised or managed by the Adviser or its affiliates.

Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock

There is no assurance that the Fund will achieve its investment objectives. Although the Fund seeks to preserve the value of your investment at $1.00 per share, if it is unable to do so, it is possible to lose money by investing in the Fund. The principal risks of investing in the Fund include:


•  Credit and Interest Rate Risk. Credit risk refers to the possibility that the issuer or guarantor of a security will be unable to make interest payments and/or repay the principal on its debt. In the case of revenue bonds, notes or commercial paper, for example, the credit risk is the possibility that the user fees from a project or other specified revenue sources are insufficient to meet interest and/or principal payment obligations. Interest rate risk refers to fluctuations in the value of a debt security resulting from changes in the general level of interest rates. The historically low interest rate environment increases the risk associated with rising interest rates.


•  Tender Option Bonds and Custodial Receipts. The risks of tender option bonds and custodial receipts include the risk that the owner of such instruments may not be considered the owner for federal income tax purposes and thus will not be entitled to treat such interest as exempt from federal income tax. Additionally, the occurrence of certain defaults or a credit rating downgrade on the underlying security may impair the ability to tender the bond or receipt back to the third-party provider of the demand option, thus causing the bond or receipt to become illiquid.


•  Investment Companies. An investment in an investment company is subject to the underlying risks of that investment company's portfolio securities. In addition to the Fund's fees and expenses, the Fund generally would bear its share of the investment company's fees and expenses.


Shares of the Fund are not bank deposits and are not guaranteed or insured by the Federal Deposit Insurance Corporation or any other government agency.

Risk Money Market Fund [Text] rr_RiskMoneyMarketFund Although the Fund seeks to preserve the value of your investment at $1.00 per share, if it is unable to do so, it is possible to lose money by investing in the Fund.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution Shares of the Fund are not bank deposits and are not guaranteed or insured by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Past Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock

The bar chart and table below provide some indication of the risks of investing in the Fund by showing changes in the Fund's performance from year to year and by showing the Fund's average annual returns for the one, five and 10 year periods. The Fund's past performance does not indicate how the Fund will perform in the future.

Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The bar chart and table below provide some indication of the risks of investing in the Fund by showing changes in the Fund's performance from year to year and by showing the Fund's average annual returns for the one, five and 10 year periods.
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The Fund's past performance does not indicate how the Fund will perform in the future.
Bar Chart [Heading] rr_BarChartHeading Annual Total Returns-Calendar Years
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock

The year-to-date total return as of September 30, 2013 was 0.01%.


High Quarter

 

6/30/07

   

0.83

%

 

Low Quarter

 

3/31/11

   

0.00

%

 
Year to Date Return, Label rr_YearToDateReturnLabel year-to-date total return
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Sep. 30, 2013
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 0.01%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel High Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2007
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 0.83%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Low Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2011
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn 0.00%
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock

For the Fund's most recent 7-day annualized yield you may call toll-free (800) 548-7786.

Caption rr_AverageAnnualReturnCaption Average Annual Total Returns (as of December 31, 2012)
Money Market Seven Day Yield Phone rr_MoneyMarketSevenDayYieldPhone (800) 548-7786
ACTIVE ASSETS TAX FREE TRUST
  
Risk/Return: rr_RiskReturnAbstract  
Advisory Fee rr_ManagementFeesOverAssets 0.24%
Distribution (12b-1) Fee rr_DistributionAndService12b1FeesOverAssets 0.10%
Other Expenses rr_OtherExpensesOverAssets 0.07%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.41%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.25%) [1]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements rr_NetExpensesOverAssets 0.16% [1]
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 16
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 52
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 90
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 205
Annual Return 2003 rr_AnnualReturn2003 0.57%
Annual Return 2004 rr_AnnualReturn2004 0.74%
Annual Return 2005 rr_AnnualReturn2005 1.94%
Annual Return 2006 rr_AnnualReturn2006 3.01%
Annual Return 2007 rr_AnnualReturn2007 3.28%
Annual Return 2008 rr_AnnualReturn2008 1.73%
Annual Return 2009 rr_AnnualReturn2009 0.04%
Annual Return 2010 rr_AnnualReturn2010 0.01%
Annual Return 2011 rr_AnnualReturn2011 0.01%
Annual Return 2012 rr_AnnualReturn2012 0.01%
Average Annual Returns, Past 1 Year rr_AverageAnnualReturnYear01 0.01%
Average Annual Returns, Past 5 Years rr_AverageAnnualReturnYear05 0.36%
Average Annual Returns, Past 10 Years rr_AverageAnnualReturnYear10 1.13%
[1] The Fund's "Distributor," Morgan Stanley Distribution, Inc., its "Adviser," Morgan Stanley Investment Management Inc., and its "Administrator," Morgan Stanley Services Company Inc., have agreed to waive and/or reimburse all or a portion of the Fund's distribution fee, advisory fee and administration fee, respectively, to the extent that total expenses exceed total income on a daily basis. These fee waivers and/or expense reimbursements will continue for at least one year or until such time as the Fund's Board of Trustees acts to discontinue all or a portion of such waivers and/or expense reimbursements when it deems such action appropriate.