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Label Element Value
Prospectus [Line Items] rr_ProspectusLineItems  
Document Type dei_DocumentType 485BPOS
Document Period End Date dei_DocumentPeriodEndDate Dec. 31, 2022
Entity Registrant Name dei_EntityRegistrantName Putnam Money Market Fund
Entity Central Index Key dei_EntityCentralIndexKey 0000081248
Entity Inv Company Type dei_EntityInvCompanyType N-1A
Amendment Flag dei_AmendmentFlag false
Putnam Money Market Fund  
Prospectus [Line Items] rr_ProspectusLineItems  
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 tables describe the fees and expenses you may pay if you buy, hold and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below. Information about sales charge discounts is available from your financial professional and in the Appendix to the fund’s prospectus.

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)
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
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock

Investments

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 and characteristics of the issuer or counterparty, 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 value of investments in the fund’s portfolio may fall or fail to rise over extended periods of time for a variety of reasons, including general economic, political or financial market conditions, investor sentiment and market perceptions, government actions, geopolitical events or changes, outbreaks of infectious illnesses or other widespread public health issues, and factors related to a specific issuer, geography, industry or sector. These and other factors may lead to increased volatility and reduced liquidity in the fund’s portfolio holdings, may negatively impact the fund’s performance, and may exacerbate other risks to which the fund is subject.

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. To the extent that the fund invests significantly in a particular industry, it runs an increased risk of loss if economic or other developments affecting that industry cause the prices of related money market investments to fall. 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. Foreign investments, including money market instruments of foreign issuers that are denominated in U.S. dollars, involve certain special risks, such as unfavorable political and legal developments, limited financial information, and economic and financial instability.

There is no guarantee that the investment techniques, analyses, or judgments that we apply in making investment decisions for the fund will produce the intended outcome or that the investments we select for the fund will perform as well as other securities that were not selected for the fund. We, or the fund’s other service providers, may experience disruptions or operating errors that could negatively impact the fund.

The fund may not achieve its goal, and it 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 Price Fluctuates [Text] rr_RiskMoneyMarketFundPriceFluctuates The values of money market investments usually rise and fall in response to changes in interest rates.
Risk Money Market Fund May Not Preserve Dollar [Text] rr_RiskMoneyMarketFundMayNotPreserveDollar Foreign investments, including money market instruments of foreign issuers that are denominated in U.S. dollars, involve certain special risks, such as unfavorable political and legal developments, limited financial information, and economic and financial instability.
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.
Risk Money Market Fund Sponsor May Not Provide Support [Text] rr_RiskMoneyMarketFundSponsorMayNotProvideSupport 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.
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
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best calendar quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Dec. 31, 2022
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 0.84%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst calendar quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2022
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn none
Performance Table Heading rr_PerformanceTableHeading Average annual total returns (for periods ending 12/31/22)
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock

Class B and C share performance reflects conversion to class A shares after eight years.

Putnam Money Market Fund | Class A  
Prospectus [Line Items] rr_ProspectusLineItems  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther 1.00% [1]
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.28%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.19%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 0.47%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 48
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 151
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 263
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 591
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%
Annual Return 2018 rr_AnnualReturn2018 1.59%
Annual Return 2019 rr_AnnualReturn2019 1.93%
Annual Return 2020 rr_AnnualReturn2020 0.35%
Annual Return 2021 rr_AnnualReturn2021 0.01%
Annual Return 2022 rr_AnnualReturn2022 1.41%
1 Year rr_AverageAnnualReturnYear01 1.41%
5 Years rr_AverageAnnualReturnYear05 1.05%
10 Years rr_AverageAnnualReturnYear10 0.60%
Putnam Money Market Fund | Class B  
Prospectus [Line Items] rr_ProspectusLineItems  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther 5.00% [1]
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.28%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.19%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 0.47%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 548 [2]
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 451 [2]
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 463 [2]
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 591 [2]
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 48
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 151
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 263
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 591
1 Year rr_AverageAnnualReturnYear01 (3.59%)
5 Years rr_AverageAnnualReturnYear05 0.67%
10 Years rr_AverageAnnualReturnYear10 0.58%
Putnam Money Market Fund | Class C  
Prospectus [Line Items] rr_ProspectusLineItems  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther 1.00% [1]
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.28%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.19%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 0.47%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 148
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 151
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 263
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 591
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 48
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 151
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 263
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 591
1 Year rr_AverageAnnualReturnYear01 0.41%
5 Years rr_AverageAnnualReturnYear05 1.05%
10 Years rr_AverageAnnualReturnYear10 0.58%
Putnam Money Market Fund | Class R  
Prospectus [Line Items] rr_ProspectusLineItems  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther none
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.28%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.19%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 0.47%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 48
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 151
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 263
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 591
1 Year rr_AverageAnnualReturnYear01 1.41%
5 Years rr_AverageAnnualReturnYear05 1.05%
10 Years rr_AverageAnnualReturnYear10 0.58%
[1] A deferred sales charge on class A, B and C 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] 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).