Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

PUTNAM MONEY MARKET FUND

Prospectus Date

rr_ProspectusDate

Jan. 30, 2015

PUTNAM TAX EXEMPT MONEY MARKET FUND | PUTNAM TAX EXEMPT MONEY MARKET FUND

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

PUTNAM TAX EXEMPT MONEY MARKET FUND

Objective [Heading]

rr_ObjectiveHeading

Goal

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

Putnam Tax Exempt Money Market Fund seeks as high a rate of current income, exempt from federal income tax, 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)

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

January 30, 2016

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. Only the first year of each period in the example takes into account the expense reimbursement described above. Your actual costs may be higher or lower.

Strategy [Heading]

rr_StrategyHeading

Investments

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

We invest mainly in money market instruments that pay interest that is exempt from federal income tax but may be subject to the federal alternative minimum tax (AMT), are high quality and have short-term maturities. 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 effects of inflation may erode the value of your investment over time. Although the fund seeks to preserve the value of your investment at $1.00 per share, deterioration in the credit quality of issuers whose securities the fund holds or an increase in interest rates may impair the value of your investment, and it is possible to lose money by investing in the fund.

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). 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 amount of information about issuers of tax exempt debt may not be as extensive as that which is available about companies whose securities are publicly traded. Interest the fund receives might be taxable.

The fund may not achieve its goal, and it is not intended to be a complete investment program. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Risk Lose Money [Text]

rr_RiskLoseMoney

it is possible to 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, deterioration in the credit quality of issuers whose securities the fund holds or an increase in interest rates may impair the value of your investment, and it is possible to lose money by investing in the fund.

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 Closing [Text Block]

rr_BarChartClosingTextBlock

Best calendar quarter Q2 2007	0.81%
Worst calendar quarter Q4 2014	0.00%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Best calendar quarter

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2007

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.81%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Worst calendar quarter

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2014

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

0.00%

Performance Table Heading

rr_PerformanceTableHeading

Average annual total returns (for periods ending 12/31/14)

PUTNAM TAX EXEMPT MONEY MARKET FUND | PUTNAM TAX EXEMPT MONEY MARKET FUND | Class A

Risk/Return:

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

0.28%

Distribution and service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.29%

Total annual fund operating expenses

rr_ExpensesOverAssets

0.57%

Expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

(0.04%)

^[2]

Total annual fund operating expenses after expense reimbursement

rr_NetExpensesOverAssets

0.53%

Expenses Deferred Charges [Text Block]

rr_ExpensesDeferredChargesTextBlock

A deferred sales charge on class A shares may apply to certain redemptions of shares purchased by exchange from another Putnam fund.

1 year

rr_ExpenseExampleYear01

54

3 years

rr_ExpenseExampleYear03

179

5 years

rr_ExpenseExampleYear05

314

10 years

rr_ExpenseExampleYear10

710

1 year

rr_ExpenseExampleNoRedemptionYear01

54

3 years

rr_ExpenseExampleNoRedemptionYear03

179

5 years

rr_ExpenseExampleNoRedemptionYear05

314

10 years

rr_ExpenseExampleNoRedemptionYear10

710

2005

rr_AnnualReturn2005

1.86%

2006

rr_AnnualReturn2006

2.93%

2007

rr_AnnualReturn2007

3.14%

2008

rr_AnnualReturn2008

1.74%

2009

rr_AnnualReturn2009

0.07%

2010

rr_AnnualReturn2010

0.04%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

1 year

rr_AverageAnnualReturnYear01

0.01%

5 years

rr_AverageAnnualReturnYear05

0.02%

10 years

rr_AverageAnnualReturnYear10

0.97%

[1] A deferred sales charge on class A shares may apply to certain redemptions of shares purchased by exchange from another Putnam fund.

[2] Reflects Putnam Investment Management, LLC's contractual obligation to limit certain fund expenses through January 30, 2016. This obligation may be modified or discontinued only with approval of the Board of Trustees.