Label	Element	Value
Prospectus [Line Items]	rr_ProspectusLineItems
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Dec. 31, 2019
Entity Registrant Name	dei_EntityRegistrantName	PUTNAM MONEY MARKET FUND
Entity Central Index Key	dei_EntityCentralIndexKey	0000081248
Amendment Flag	dei_AmendmentFlag	false
Entity Inv Company Type	dei_EntityInvCompanyType	N-1A
Trading Symbol	dei_TradingSymbol	PDDXX
Document Creation Date	dei_DocumentCreationDate	Jan. 27, 2020
Document Effective Date	dei_DocumentEffectiveDate	Jan. 30, 2020
Prospectus Date	rr_ProspectusDate	Jan. 30, 2020
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 and hold shares of the fund. Information about sales charge discounts is available from your financial advisor 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, 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. 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. 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 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 Impose Fees or Suspend Sales [Text]	rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales	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 [Text]	rr_RiskNotInsured	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
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 2019  0.54% Worst calender quarter Q3 2016  0.00%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best calendar quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2019
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.54%
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 12/31/19)
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
Prospectus [Line Items]	rr_ProspectusLineItems
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	0.28%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none	[2]
Other expenses	rr_OtherExpensesOverAssets	0.20%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.48%
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	$ 49
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	154
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	269
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	$ 604
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%
Annual Return 2018	rr_AnnualReturn2018	1.59%
Annual Return 2019	rr_AnnualReturn2019	1.93%
Label	rr_AverageAnnualReturnLabel	before taxes
1 Year	rr_AverageAnnualReturnYear01	1.93%
5 Years	rr_AverageAnnualReturnYear05	0.84%
10 Years	rr_AverageAnnualReturnYear10	0.43%
Putnam Money Market Fund | Class B
Prospectus [Line Items]	rr_ProspectusLineItems
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	0.28%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none	[2]
Other expenses	rr_OtherExpensesOverAssets	0.20%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.48%
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	$ 549	[3]
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	454	[3]
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	469	[3]
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	604	[3]
Expense Example, No Redemption, 1 Year	rr_ExpenseExampleNoRedemptionYear01	49
Expense Example, No Redemption, 3 Years	rr_ExpenseExampleNoRedemptionYear03	154
Expense Example, No Redemption, 5 Years	rr_ExpenseExampleNoRedemptionYear05	269
Expense Example, No Redemption, 10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 604
Label	rr_AverageAnnualReturnLabel	before taxes
1 Year	rr_AverageAnnualReturnYear01	(3.07%)
5 Years	rr_AverageAnnualReturnYear05	0.42%
10 Years	rr_AverageAnnualReturnYear10	0.42%
Putnam Money Market Fund | Class C
Prospectus [Line Items]	rr_ProspectusLineItems
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	0.28%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none	[2]
Other expenses	rr_OtherExpensesOverAssets	0.20%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.48%
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	$ 149	[3]
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	154	[3]
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	269	[3]
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	604	[3]
Expense Example, No Redemption, 1 Year	rr_ExpenseExampleNoRedemptionYear01	49
Expense Example, No Redemption, 3 Years	rr_ExpenseExampleNoRedemptionYear03	154
Expense Example, No Redemption, 5 Years	rr_ExpenseExampleNoRedemptionYear05	269
Expense Example, No Redemption, 10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 604
Label	rr_AverageAnnualReturnLabel	before taxes
1 Year	rr_AverageAnnualReturnYear01	0.93%
5 Years	rr_AverageAnnualReturnYear05	0.81%
10 Years	rr_AverageAnnualReturnYear10	0.42%
Putnam Money Market Fund | Class R
Prospectus [Line Items]	rr_ProspectusLineItems
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	0.28%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none	[2]
Other expenses	rr_OtherExpensesOverAssets	0.20%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.48%
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	$ 49
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	154
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	269
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	$ 604
Label	rr_AverageAnnualReturnLabel	before taxes
1 Year	rr_AverageAnnualReturnYear01	1.93%
5 Years	rr_AverageAnnualReturnYear05	0.81%
10 Years	rr_AverageAnnualReturnYear10	0.42%

[1] A deferred sales charge on class A, B and C shares may apply to certain redemptions of shares purchased by exchange from another

[2] 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 and class R 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 and class R shares.

[3] Reflects assessment of deferred sales charge assuming class B or C shares were acquired by exchange from one or more other Putnam funds immediately after purchase of shares from such other fund(s).