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Risk/Return:rr_RiskReturnAbstract 
Document Typedei_DocumentType485BPOS
Document Period End Datedei_DocumentPeriodEndDateAug. 31, 2011
Registrant Namedei_EntityRegistrantNameFIDELITY MONEY MARKET TRUST
Central Index Keydei_EntityCentralIndexKey0000276516
Amendment Flagdei_AmendmentFlagfalse
Document Creation Datedei_DocumentCreationDateOct. 27, 2011
Document Effective Datedei_DocumentEffectiveDateOct. 29, 2011
Prospectus Daterr_ProspectusDateOct. 29, 2011
Retirement Government Money Market Portfolio
 
Risk/Return:rr_RiskReturnAbstract 
Risk/Return [Heading]rr_RiskReturnHeading
Fund Summary
Fund:
Retirement Government Money Market Portfolio
Objective [Heading]rr_ObjectiveHeadingInvestment Objective
Objective, Primary [Text Block]rr_ObjectivePrimaryTextBlockThe fund seeks as high a level of current income as is consistent with the security of principal and liquidity.
Fee Tablefmmt276516_FundFeesAndExpensesAbstract 
Expense [Heading]rr_ExpenseHeadingFee Table
Expense Narrative [Text Block]rr_ExpenseNarrativeTextBlockThe following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.
Shareholder feesrr_ShareholderFeesAbstract 
Shareholder Fees Caption [Text]rr_ShareholderFeesCaptionShareholder fees (fees paid directly from your investment)
Annual fund operating expensesrr_OperatingExpensesAbstract 
Operating Expenses Caption [Text]rr_OperatingExpensesCaptionAnnual fund operating expenses (expenses that you pay each year as a % of the value of your investment)
Expense Examplerr_ExpenseExampleAbstract 
Expense Example Narrative [Text Block]rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other mutual funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Strategy [Heading]rr_StrategyHeadingPrincipal Investment Strategies
Strategy Narrative [Text Block]rr_StrategyNarrativeTextBlock
  • Normally investing at least 80% of assets in U.S. Government securities and repurchase agreements for those securities.
  • Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury.
  • Potentially entering into reverse repurchase agreements.
  • Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, and diversification of investments.
Risk [Heading]rr_RiskHeadingPrincipal Investment Risks
Risk Narrative [Text Block]rr_RiskNarrativeTextBlock
  • Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.
  • Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

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

Risk Not Insured Depository Institution [Text]rr_RiskNotInsuredDepositoryInstitutionAn investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Risk Money Market Fund [Text]rr_RiskMoneyMarketFundAlthough the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Performancefmmt276516_FundPastPerformanceAbstract 
Bar Chart and Performance Table [Heading]rr_BarChartAndPerformanceTableHeadingPerformance
Performance Narrative [Text Block]rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com for updated return information.

Performance Information Illustrates Variability of Returns [Text]rr_PerformanceInformationIllustratesVariabilityOfReturnsThe information illustrates the changes in the performance of the fund's shares from year to year.
Performance Past Does Not Indicate Future [Text]rr_PerformancePastDoesNotIndicateFuturePast performance is not an indication of future performance.
Performance Availability Website Address [Text]rr_PerformanceAvailabilityWebSiteAddresswww.fidelity.com
Annual Total Returnsrr_BarChartTableAbstract 
Bar Chart [Heading]rr_BarChartHeading

Year-by-Year Returns

Calendar Years

Bar Chart Closing [Text Block]rr_BarChartClosingTextBlock
During the periods shown in the chart:
Returns
Quarter ended
Highest Quarter Return1.39%March 31, 2001
Lowest Quarter Return0.00%March 31, 2010
Year-to-Date Return0.01%September 30, 2011
Average Annual Total Returnsrr_AverageAnnualReturnAbstract 
Performance Table Headingrr_PerformanceTableHeadingAverage Annual Returns
For the periods ended
December 31, 2010
Class: Retirement Government Money Market Portfolio | Retirement Government Money Market Portfolio
 
Shareholder feesrr_ShareholderFeesAbstract 
Shareholder fees (fees paid directly from your investment)rr_ShareholderFeeOther none
Annual fund operating expensesrr_OperatingExpensesAbstract 
Management feerr_ManagementFeesOverAssets0.42%
Distribution and/or Service (12b-1) feesrr_DistributionAndService12b1FeesOverAssets none
Other expensesrr_OtherExpensesOverAssets none
Total annual fund operating expensesrr_ExpensesOverAssets0.42%
Expense Examplerr_ExpenseExampleAbstract 
1 yearrr_ExpenseExampleYear0143
3 yearsrr_ExpenseExampleYear03135
5 yearsrr_ExpenseExampleYear05235
10 yearsrr_ExpenseExampleYear10530
Annual Total Returnsrr_BarChartTableAbstract 
2001rr_AnnualReturn20014.09%
2002rr_AnnualReturn20021.54%
2003rr_AnnualReturn20030.83%
2004rr_AnnualReturn20041.03%
2005rr_AnnualReturn20052.88%
2006rr_AnnualReturn20064.74%
2007rr_AnnualReturn20074.93%
2008rr_AnnualReturn20082.37%
2009rr_AnnualReturn20090.29%
2010rr_AnnualReturn20100.01%
Highest Quarterly Return, Labelrr_HighestQuarterlyReturnLabelHighest Quarter Return
Highest Quarterly Returnrr_BarChartHighestQuarterlyReturn1.39%
Highest Quarterly Return, Daterr_BarChartHighestQuarterlyReturnDateMar. 31, 2001
Lowest Quarterly Return, Labelrr_LowestQuarterlyReturnLabelLowest Quarter Return
Lowest Quarterly Returnrr_BarChartLowestQuarterlyReturn0.00%
Lowest Quarterly Return, Daterr_BarChartLowestQuarterlyReturnDateMar. 31, 2010
Year to Date Return, Labelrr_YearToDateReturnLabelYear-to-Date Return
Bar Chart, Year to Date Returnrr_BarChartYearToDateReturn0.01%
Bar Chart, Year to Date Return, Daterr_BarChartYearToDateReturnDateSep. 30, 2011
Average Annual Total Returnsrr_AverageAnnualReturnAbstract 
Labelrr_AverageAnnualReturnLabelRetirement Government Money Market Portfolio
Past 1 yearrr_AverageAnnualReturnYear010.01%
Past 5 yearsrr_AverageAnnualReturnYear052.45%
Past 10 yearsrr_AverageAnnualReturnYear102.26%
Retirement Money Market Portfolio
 
Risk/Return:rr_RiskReturnAbstract 
Risk/Return [Heading]rr_RiskReturnHeading
Fund Summary
Fund:
Retirement Money Market Portfolio
Objective [Heading]rr_ObjectiveHeadingInvestment Objective
Objective, Primary [Text Block]rr_ObjectivePrimaryTextBlockThe fund seeks to obtain as high a level of current income as is consistent with the preservation of capital and liquidity.
Fee Tablefmmt276516_FundFeesAndExpensesAbstract 
Expense [Heading]rr_ExpenseHeadingFee Table
Expense Narrative [Text Block]rr_ExpenseNarrativeTextBlockThe following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.
Shareholder feesrr_ShareholderFeesAbstract 
Shareholder Fees Caption [Text]rr_ShareholderFeesCaptionShareholder fees (fees paid directly from your investment)
Annual fund operating expensesrr_OperatingExpensesAbstract 
Operating Expenses Caption [Text]rr_OperatingExpensesCaptionAnnual fund operating expenses (expenses that you pay each year as a % of the value of your investment)
Expense Examplerr_ExpenseExampleAbstract 
Expense Example Narrative [Text Block]rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other mutual funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Strategy [Heading]rr_StrategyHeadingPrincipal Investment Strategies
Strategy Narrative [Text Block]rr_StrategyNarrativeTextBlock
  • Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers and repurchase agreements.
  • Potentially entering into reverse repurchase agreements.
  • Investing more than 25% of total assets in the financial services industries.
  • Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, and diversification of investments.
Strategy Portfolio Concentration [Text]rr_StrategyPortfolioConcentration
  • Investing more than 25% of total assets in the financial services industries.
Risk [Heading]rr_RiskHeadingPrincipal Investment Risks
Risk Narrative [Text Block]rr_RiskNarrativeTextBlock
  • Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.
  • Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.
  • Financial Services Exposure. 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.
  • Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

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

Risk Not Insured Depository Institution [Text]rr_RiskNotInsuredDepositoryInstitutionAn investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Risk Money Market Fund [Text]rr_RiskMoneyMarketFundAlthough the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Performancefmmt276516_FundPastPerformanceAbstract 
Bar Chart and Performance Table [Heading]rr_BarChartAndPerformanceTableHeadingPerformance
Performance Narrative [Text Block]rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com for updated return information.

Performance Information Illustrates Variability of Returns [Text]rr_PerformanceInformationIllustratesVariabilityOfReturnsThe information illustrates the changes in the performance of the fund's shares from year to year.
Performance Past Does Not Indicate Future [Text]rr_PerformancePastDoesNotIndicateFuturePast performance is not an indication of future performance.
Performance Availability Website Address [Text]rr_PerformanceAvailabilityWebSiteAddresswww.fidelity.com
Annual Total Returnsrr_BarChartTableAbstract 
Bar Chart [Heading]rr_BarChartHeading

Year-by-Year Returns

Calendar Years

Bar Chart Closing [Text Block]rr_BarChartClosingTextBlock
During the periods shown in the chart:
Returns
Quarter ended
Highest Quarter Return1.42%March 31, 2001
Lowest Quarter Return0.00%June 30, 2010
Year-to-Date Return0.01%September 30, 2011
Average Annual Total Returnsrr_AverageAnnualReturnAbstract 
Performance Table Headingrr_PerformanceTableHeadingAverage Annual Returns
For the periods ended
December 31, 2010
Retirement Money Market Portfolio | Class: Retirement Money Market Portfolio
 
Shareholder feesrr_ShareholderFeesAbstract 
Shareholder fees (fees paid directly from your investment)rr_ShareholderFeeOther none
Annual fund operating expensesrr_OperatingExpensesAbstract 
Management feerr_ManagementFeesOverAssets0.42%
Distribution and/or Service (12b-1) feesrr_DistributionAndService12b1FeesOverAssets none
Other expensesrr_OtherExpensesOverAssets none
Total annual fund operating expensesrr_ExpensesOverAssets0.42%
Expense Examplerr_ExpenseExampleAbstract 
1 yearrr_ExpenseExampleYear0143
3 yearsrr_ExpenseExampleYear03135
5 yearsrr_ExpenseExampleYear05235
10 yearsrr_ExpenseExampleYear10 530
Annual Total Returnsrr_BarChartTableAbstract 
2001rr_AnnualReturn20014.05%
2002rr_AnnualReturn20021.57%
2003rr_AnnualReturn20030.89%
2004rr_AnnualReturn20041.07%
2005rr_AnnualReturn20052.96%
2006rr_AnnualReturn20064.82%
2007rr_AnnualReturn20075.12%
2008rr_AnnualReturn20082.93%
2009rr_AnnualReturn20090.63%
2010rr_AnnualReturn20100.02%
Highest Quarterly Return, Labelrr_HighestQuarterlyReturnLabelHighest Quarter Return
Highest Quarterly Returnrr_BarChartHighestQuarterlyReturn1.42%
Highest Quarterly Return, Daterr_BarChartHighestQuarterlyReturnDateMar. 31, 2001
Lowest Quarterly Return, Labelrr_LowestQuarterlyReturnLabelLowest Quarter Return
Lowest Quarterly Returnrr_BarChartLowestQuarterlyReturn0.00%
Lowest Quarterly Return, Daterr_BarChartLowestQuarterlyReturnDateJun. 30, 2010
Year to Date Return, Labelrr_YearToDateReturnLabelYear-to-Date Return
Bar Chart, Year to Date Returnrr_BarChartYearToDateReturn0.01%
Bar Chart, Year to Date Return, Daterr_BarChartYearToDateReturnDateSep. 30, 2011
Average Annual Total Returnsrr_AverageAnnualReturnAbstract 
Labelrr_AverageAnnualReturnLabelRetirement Money Market Portfolio
Past 1 yearrr_AverageAnnualReturnYear010.02%
Past 5 yearsrr_AverageAnnualReturnYear052.68%
Past 10 yearsrr_AverageAnnualReturnYear102.39%