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Label Element Value
Risk Return Abstract rr_RiskReturnAbstract  
Document Type dei_DocumentType 485BPOS
Document Period End Date dei_DocumentPeriodEndDate Dec. 31, 2017
Registrant Name dei_EntityRegistrantName PUTNAM MONEY MARKET FUND
Central Index Key dei_EntityCentralIndexKey 0000081248
Amendment Flag dei_AmendmentFlag false
Trading Symbol dei_TradingSymbol PDDXX
Document Creation Date dei_DocumentCreationDate Jan. 30, 2018
Document Effective Date dei_DocumentEffectiveDate Jan. 30, 2018
Prospectus Date rr_ProspectusDate Jan. 30, 2018
Putnam Money Market Fund  
Risk Return Abstract rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Fund Summary
Objective [Heading] rr_ObjectiveHeading Goal
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock Putnam Money Market Fund seeks as high a rate of current income as Putnam Investment Management, LLC believes is consistent with preservation of capital and maintenance of liquidity.
Expense [Heading] rr_ExpenseHeading Fees and expenses
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock The following table describes the fees and expenses you may pay if you buy and hold shares of the fund.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption Shareholder fees (fees paid directly from your investment)
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual fund operating expenses
(expenses you pay each year as a percentage of the value of your investment)
Expenses Restated to Reflect Current [Text] rr_ExpensesRestatedToReflectCurrent Restated to reflect current fees. The fund's Board of Trustees determined to limit payments under the distribution and service (12b-1) plans in place with respect to class B, class C, class M, class R and class T1 shares to 0.00% of average net assets effective as of the close of business on March 31, 2017. Beginning on April 1, 2017, the fund no longer makes payments under the distribution and service (12b-1) plans in place with respect to class B, class C, class M, class R and class T1 shares.
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock The following hypothetical example is intended to help you compare the cost of investing in the fund with the cost of investing in other funds. It assumes that you invest $10,000 in the fund for the time periods indicated and then redeem or hold all your shares at the end of those periods. It assumes a 5% return on your investment each year and that the fund’s operating expenses remain the same. Your actual costs may be higher or lower.
Strategy [Heading] rr_StrategyHeading Investments, risks, and performance

Investments
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock We invest mainly in money market instruments that are high quality and have short-term maturities. We invest significantly in certificates of deposit, commercial paper (including asset-backed commercial paper), U.S. government debt and repurchase agreements, corporate obligations and time deposits. We may also invest in U.S. dollar denominated foreign securities of these types. We may consider, among other factors, credit and interest rate risks, as well as general market conditions, when deciding whether to buy or sell investments.
Risk [Heading] rr_RiskHeading Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock The values of money market investments usually rise and fall in response to changes in interest rates. Interest rate risk is generally lowest for investments with short maturities (a significant part of the fund’s investments). Changes in the financial condition of an issuer or counterparty, changes in specific economic or political conditions that affect a particular type of issuer, and changes in general economic or political conditions can increase the risk of default by an issuer or counterparty, which can affect a security’s or instrument’s credit quality or value. Although the fund only buys high quality investments, investments backed by a letter of credit have the risk that the provider of the letter of credit will not be able to fulfill its obligations to the issuer. The effects of inflation may erode the value of your investment over time.

The fund may not achieve its goal, and is not intended to be a complete investment program. You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund’s liquidity falls below certain required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. The fund’s sponsor has no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.
Risk Lose Money [Text] rr_RiskLoseMoney You could lose money by investing in the fund.
Risk Money Market Fund [Text] rr_RiskMoneyMarketFund Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund’s liquidity falls below certain required minimums because of market conditions or other factors.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The performance information below gives some indication of the risks associated with an investment in the fund by showing the fund’s performance year to year and over time. Please remember that past performance is not necessarily an indication of future results. Monthly performance figures for the fund are available at putnam.com.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The performance information below gives some indication of the risks associated with an investment in the fund by showing the fund’s performance year to year and over time.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress putnam.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture Please remember that past performance is not necessarily an indication of future results.
Bar Chart [Heading] rr_BarChartHeading Annual total returns for class A shares
Bar Chart Does Not Reflect Sales Loads [Text] rr_BarChartDoesNotReflectSalesLoads The bar chart does not reflect the impact of sales charges. If it did, performance would be lower.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock Best calendar quarter
Q1 2008   0.92%

Worst calendar quarter
Q3 2016  0.00%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best calendar quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 30, 2008
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 0.92%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst calendar quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2016
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn none
Performance Table Heading rr_PerformanceTableHeading Average annual total returns (for periods ended 12/31/17)
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock Class B share performance reflects conversion to class A shares after eight years.
Putnam Money Market Fund | Class A  
Risk Return Abstract rr_RiskReturnAbstract  
Maximum sales charge (load) imposed on purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum deferred sales charge (load) (as a percentage of original purchase price or redemption proceeds, whichever is lower) rr_MaximumDeferredSalesChargeOverOther 1.00% [1]
Management fees rr_ManagementFeesOverAssets 0.29%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets [2]
Other expenses rr_OtherExpensesOverAssets 0.21%
Total annual fund operating expenses rr_ExpensesOverAssets 0.50%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 51
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 160
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 280
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 628
Annual Return 2008 rr_AnnualReturn2008 2.71%
Annual Return 2009 rr_AnnualReturn2009 0.29%
Annual Return 2010 rr_AnnualReturn2010 0.05%
Annual Return 2011 rr_AnnualReturn2011 0.01%
Annual Return 2012 rr_AnnualReturn2012 0.01%
Annual Return 2013 rr_AnnualReturn2013 0.01%
Annual Return 2014 rr_AnnualReturn2014 0.01%
Annual Return 2015 rr_AnnualReturn2015 0.01%
Annual Return 2016 rr_AnnualReturn2016 0.04%
Annual Return 2017 rr_AnnualReturn2017 0.64%
1 Year rr_AverageAnnualReturnYear01 0.64%
5 Years rr_AverageAnnualReturnYear05 0.14%
10 Years rr_AverageAnnualReturnYear10 0.37%
Putnam Money Market Fund | Class B  
Risk Return Abstract rr_RiskReturnAbstract  
Maximum sales charge (load) imposed on purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum deferred sales charge (load) (as a percentage of original purchase price or redemption proceeds, whichever is lower) rr_MaximumDeferredSalesChargeOverOther 5.00% [1]
Management fees rr_ManagementFeesOverAssets 0.29%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets [2]
Other expenses rr_OtherExpensesOverAssets 0.21%
Total annual fund operating expenses rr_ExpensesOverAssets 0.50%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 551 [3]
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 460 [3]
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 480 [3]
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 628 [3]
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 51
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 160
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 280
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 628
1 Year rr_AverageAnnualReturnYear01 (4.45%)
5 Years rr_AverageAnnualReturnYear05 (0.28%)
10 Years rr_AverageAnnualReturnYear10 0.31%
Putnam Money Market Fund | Class C  
Risk Return Abstract rr_RiskReturnAbstract  
Maximum sales charge (load) imposed on purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum deferred sales charge (load) (as a percentage of original purchase price or redemption proceeds, whichever is lower) rr_MaximumDeferredSalesChargeOverOther 1.00% [1]
Management fees rr_ManagementFeesOverAssets 0.29%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets [2]
Other expenses rr_OtherExpensesOverAssets 0.21%
Total annual fund operating expenses rr_ExpensesOverAssets 0.50%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 151
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 160
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 280
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 628
1 Year rr_AverageAnnualReturnYear01 (0.45%)
5 Years rr_AverageAnnualReturnYear05 0.12%
10 Years rr_AverageAnnualReturnYear10 0.29%
Putnam Money Market Fund | Class M  
Risk Return Abstract rr_RiskReturnAbstract  
Maximum sales charge (load) imposed on purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum deferred sales charge (load) (as a percentage of original purchase price or redemption proceeds, whichever is lower) rr_MaximumDeferredSalesChargeOverOther none
Management fees rr_ManagementFeesOverAssets 0.29%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets [2]
Other expenses rr_OtherExpensesOverAssets 0.21%
Total annual fund operating expenses rr_ExpensesOverAssets 0.50%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 51
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 160
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 280
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 628
1 Year rr_AverageAnnualReturnYear01 0.60%
5 Years rr_AverageAnnualReturnYear05 0.13%
10 Years rr_AverageAnnualReturnYear10 0.35%
Putnam Money Market Fund | Class R  
Risk Return Abstract rr_RiskReturnAbstract  
Maximum sales charge (load) imposed on purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum deferred sales charge (load) (as a percentage of original purchase price or redemption proceeds, whichever is lower) rr_MaximumDeferredSalesChargeOverOther none
Management fees rr_ManagementFeesOverAssets 0.29%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets [2]
Other expenses rr_OtherExpensesOverAssets 0.21%
Total annual fund operating expenses rr_ExpensesOverAssets 0.50%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 51
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 160
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 280
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 628
1 Year rr_AverageAnnualReturnYear01 0.55%
5 Years rr_AverageAnnualReturnYear05 0.12%
10 Years rr_AverageAnnualReturnYear10 0.29%
Putnam Money Market Fund | Class T1  
Risk Return Abstract rr_RiskReturnAbstract  
Maximum sales charge (load) imposed on purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum deferred sales charge (load) (as a percentage of original purchase price or redemption proceeds, whichever is lower) rr_MaximumDeferredSalesChargeOverOther 1.00% [1]
Management fees rr_ManagementFeesOverAssets 0.29%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets [2]
Other expenses rr_OtherExpensesOverAssets 0.21%
Total annual fund operating expenses rr_ExpensesOverAssets 0.50%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 51
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 160
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 280
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 628
1 Year rr_AverageAnnualReturnYear01 0.58%
5 Years rr_AverageAnnualReturnYear05 0.12%
10 Years rr_AverageAnnualReturnYear10 0.33%
[1] A deferred sales charge on class A, B, C, and T1 shares may apply to certain redemptions of shares purchased by exchange from another Putnam fund. The rate of the deferred sales charge for class B shares will be determined based on the time between the original purchase of class B shares from the other Putnam fund(s) and the date of redemption of shares from this fund.
[2] Restated to reflect current fees. The fund's Board of Trustees determined to limit payments under the distribution and service (12b-1) plans in place with respect to class B, class C, class M, class R and class T1 shares to 0.00% of average net assets effective as of the close of business on March 31, 2017. Beginning on April 1, 2017, the fund no longer makes payments under the distribution and service (12b-1) plans in place with respect to class B, class C, class M, class R and class T1 shares.
[3] Reflects assessment of deferred sales charge assuming class B shares were acquired by exchange from one or more other Putnam funds immediately after purchase of shares from such other fund(s).