Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Aug. 31, 2014
Registrant Name	dei_EntityRegistrantName	Vanguard Money Market Reserves
Central Index Key	dei_EntityCentralIndexKey	0000106830
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Dec. 19, 2014
Document Effective Date	dei_DocumentEffectiveDate	Dec. 19, 2014
Vanguard Federal Money Market Fund | Vanguard Federal Money Market Fund - Investor Shares
Risk/Return:	rr_RiskReturnAbstract
Sales charge (load) imposed on purchases	rr_MaximumCumulativeSalesChargeOverOfferingPrice	none
Purchase Fee	rr_MaximumCumulativeSalesChargeOverOther	none
Sales charge (load) imposed on reinvested dividends	rr_MaximumSalesChargeOnReinvestedDividendsAndDistributionsOverOther	none
Redemption Fee	rr_RedemptionFeeOverRedemption	none
Account Service Fee (for certain fund account balances below $10,000)	rr_MaximumAccountFee	20
Management Fees	rr_ManagementFeesOverAssets	0.09%
12b-1 Distribution Fee	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.02%
Total Annual Fund Operating Expenses	rr_NetExpensesOverAssets	0.11%	[1]
1 YEAR	rr_ExpenseExampleNoRedemptionYear01	11
3 YEAR	rr_ExpenseExampleNoRedemptionYear03	35
5 YEAR	rr_ExpenseExampleNoRedemptionYear05	62
10 YEAR	rr_ExpenseExampleNoRedemptionYear10	141
Annual Return Inception Date	rr_AnnualReturnInceptionDate	Jul. 13, 1981
2004	rr_AnnualReturn2004	1.08%
2005	rr_AnnualReturn2005	2.96%
2006	rr_AnnualReturn2006	4.81%
2007	rr_AnnualReturn2007	5.07%
2008	rr_AnnualReturn2008	2.53%
2009	rr_AnnualReturn2009	0.40%
2010	rr_AnnualReturn2010	0.02%
2011	rr_AnnualReturn2011	0.01%
2012	rr_AnnualReturn2012	0.01%
2013	rr_AnnualReturn2013	0.02%
Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2014
Year to Date Return	rr_BarChartYearToDateReturn	0.01%
Bar Chart Highest Quarterly Return Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Bar Chart Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.28%
Bar Chart Lowest Quarter Return Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2013
Bar Chart Lowest Quarter Return	rr_BarChartLowestQuarterlyReturn	0.00%
One Year	rr_AverageAnnualReturnYear01	0.02%
Five Years	rr_AverageAnnualReturnYear05	0.09%
Ten Years	rr_AverageAnnualReturnYear10	1.67%
Inception Date	rr_AverageAnnualReturnInceptionDate	Jul. 13, 1981
Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Investor Shares
Risk/Return:	rr_RiskReturnAbstract
Sales charge (load) imposed on purchases	rr_MaximumCumulativeSalesChargeOverOfferingPrice	none
Purchase Fee	rr_MaximumCumulativeSalesChargeOverOther	none
Sales charge (load) imposed on reinvested dividends	rr_MaximumSalesChargeOnReinvestedDividendsAndDistributionsOverOther	none
Redemption Fee	rr_RedemptionFeeOverRedemption	none
Account Service Fee (for certain fund account balances below $10,000)	rr_MaximumAccountFee	20
Management Fees	rr_ManagementFeesOverAssets	0.13%
12b-1 Distribution Fee	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.03%
Total Annual Fund Operating Expenses	rr_NetExpensesOverAssets	0.16%	[1]
1 YEAR	rr_ExpenseExampleNoRedemptionYear01	16
3 YEAR	rr_ExpenseExampleNoRedemptionYear03	52
5 YEAR	rr_ExpenseExampleNoRedemptionYear05	90
10 YEAR	rr_ExpenseExampleNoRedemptionYear10	205
Annual Return Inception Date	rr_AnnualReturnInceptionDate	Jun. 04, 1975
2004	rr_AnnualReturn2004	1.11%
2005	rr_AnnualReturn2005	3.01%
2006	rr_AnnualReturn2006	4.88%
2007	rr_AnnualReturn2007	5.14%
2008	rr_AnnualReturn2008	2.77%
2009	rr_AnnualReturn2009	0.53%
2010	rr_AnnualReturn2010	0.06%
2011	rr_AnnualReturn2011	0.05%
2012	rr_AnnualReturn2012	0.04%
2013	rr_AnnualReturn2013	0.02%
Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2014
Year to Date Return	rr_BarChartYearToDateReturn	0.01%
Bar Chart Highest Quarterly Return Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Bar Chart Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.29%
Bar Chart Lowest Quarter Return Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2013
Bar Chart Lowest Quarter Return	rr_BarChartLowestQuarterlyReturn	0.00%
One Year	rr_AverageAnnualReturnYear01	0.02%
Five Years	rr_AverageAnnualReturnYear05	0.14%
Ten Years	rr_AverageAnnualReturnYear10	1.74%
Inception Date	rr_AverageAnnualReturnInceptionDate	Jun. 04, 1975
Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
Sales charge (load) imposed on purchases	rr_MaximumCumulativeSalesChargeOverOfferingPrice	none
Purchase Fee	rr_MaximumCumulativeSalesChargeOverOther	none
Sales charge (load) imposed on reinvested dividends	rr_MaximumSalesChargeOnReinvestedDividendsAndDistributionsOverOther	none
Redemption Fee	rr_RedemptionFeeOverRedemption	none
Account Service Fee (for certain fund account balances below $10,000)	rr_MaximumAccountFee	20
Management Fees	rr_ManagementFeesOverAssets	0.07%
12b-1 Distribution Fee	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.03%
Total Annual Fund Operating Expenses	rr_NetExpensesOverAssets	0.10%	[1]
1 YEAR	rr_ExpenseExampleNoRedemptionYear01	10
3 YEAR	rr_ExpenseExampleNoRedemptionYear03	32
5 YEAR	rr_ExpenseExampleNoRedemptionYear05	56
10 YEAR	rr_ExpenseExampleNoRedemptionYear10	128
Annual Return Inception Date	rr_AnnualReturnInceptionDate	Oct. 03, 1989
2004	rr_AnnualReturn2004	1.33%
2005	rr_AnnualReturn2005	3.21%
2006	rr_AnnualReturn2006	5.08%
2007	rr_AnnualReturn2007	5.30%
2008	rr_AnnualReturn2008	2.93%
2009	rr_AnnualReturn2009	0.68%
2010	rr_AnnualReturn2010	0.20%
2011	rr_AnnualReturn2011	0.14%
2012	rr_AnnualReturn2012	0.11%
2013	rr_AnnualReturn2013	0.06%
Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2014
Year to Date Return	rr_BarChartYearToDateReturn	0.04%
Bar Chart Highest Quarterly Return Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2006
Bar Chart Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.34%
Bar Chart Lowest Quarter Return Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2013
Bar Chart Lowest Quarter Return	rr_BarChartLowestQuarterlyReturn	0.01%
One Year	rr_AverageAnnualReturnYear01	0.06%
Five Years	rr_AverageAnnualReturnYear05	0.24%
Ten Years	rr_AverageAnnualReturnYear10	1.88%
Inception Date	rr_AverageAnnualReturnInceptionDate	Oct. 03, 1989
Citigroup 3-Month U.S. Treasury Bill Index | Vanguard Federal Money Market Fund | Vanguard Federal Money Market Fund - Investor Shares
Risk/Return:	rr_RiskReturnAbstract
One Year	rr_AverageAnnualReturnYear01	0.05%
Five Years	rr_AverageAnnualReturnYear05	0.09%
Ten Years	rr_AverageAnnualReturnYear10	1.58%
Citigroup 3-Month U.S. Treasury Bill Index | Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Investor Shares
Risk/Return:	rr_RiskReturnAbstract
One Year	rr_AverageAnnualReturnYear01	0.05%
Five Years	rr_AverageAnnualReturnYear05	0.09%
Ten Years	rr_AverageAnnualReturnYear10	1.58%
Citigroup 3-Month U.S. Treasury Bill Index | Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
One Year	rr_AverageAnnualReturnYear01	0.05%
Five Years	rr_AverageAnnualReturnYear05	0.09%
Ten Years	rr_AverageAnnualReturnYear10	1.58%
U.S. Government Money Market Funds Average | Vanguard Federal Money Market Fund | Vanguard Federal Money Market Fund - Investor Shares
Risk/Return:	rr_RiskReturnAbstract
One Year	rr_AverageAnnualReturnYear01	none
Five Years	rr_AverageAnnualReturnYear05	0.02%
Ten Years	rr_AverageAnnualReturnYear10	1.34%
Money Market Funds Average | Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Investor Shares
Risk/Return:	rr_RiskReturnAbstract
One Year	rr_AverageAnnualReturnYear01	none
Five Years	rr_AverageAnnualReturnYear05	0.03%
Ten Years	rr_AverageAnnualReturnYear10	1.37%
Institutional Money Market Funds Average | Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
One Year	rr_AverageAnnualReturnYear01	none
Five Years	rr_AverageAnnualReturnYear05	0.10%
Ten Years	rr_AverageAnnualReturnYear10	1.63%

[1] Vanguard and the Fund's Board have voluntarily agreed to temporarily limit certain net operating expenses in excess of the Fund's daily yield so as to maintain a zero or positive yield for the Fund. Vanguard and the Fund's Board may terminate the temporary expense limitation at any time.