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Average Annual Total Returns
 12 Months Ended
Aug. 31, 2014
Vanguard Federal Money Market Fund | Vanguard Federal Money Market Fund - Investor Shares  
Average Annual Returns for Periods Ended December 31, 2013  
One Year 0.02%rr_AverageAnnualReturnYear01
/ dei_LegalEntityAxis
= vanguard_S000004462Member
/ rr_ProspectusShareClassAxis
= vanguard_C000012238Member
Five Years 0.09%rr_AverageAnnualReturnYear05
/ dei_LegalEntityAxis
= vanguard_S000004462Member
/ rr_ProspectusShareClassAxis
= vanguard_C000012238Member
Ten Years 1.67%rr_AverageAnnualReturnYear10
/ dei_LegalEntityAxis
= vanguard_S000004462Member
/ rr_ProspectusShareClassAxis
= vanguard_C000012238Member
Inception Date Jul. 13, 1981
Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Investor Shares  
Average Annual Returns for Periods Ended December 31, 2013  
One Year 0.02%rr_AverageAnnualReturnYear01
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_ProspectusShareClassAxis
= vanguard_C000012239Member
Five Years 0.14%rr_AverageAnnualReturnYear05
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_ProspectusShareClassAxis
= vanguard_C000012239Member
Ten Years 1.74%rr_AverageAnnualReturnYear10
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_ProspectusShareClassAxis
= vanguard_C000012239Member
Inception Date Jun. 04, 1975
Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Institutional Shares  
Average Annual Returns for Periods Ended December 31, 2013  
One Year 0.06%rr_AverageAnnualReturnYear01
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_ProspectusShareClassAxis
= vanguard_C000012240Member
Five Years 0.24%rr_AverageAnnualReturnYear05
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_ProspectusShareClassAxis
= vanguard_C000012240Member
Ten Years 1.88%rr_AverageAnnualReturnYear10
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_ProspectusShareClassAxis
= vanguard_C000012240Member
Inception Date Oct. 03, 1989
Citigroup 3-Month U.S. Treasury Bill Index | Vanguard Federal Money Market Fund | Vanguard Federal Money Market Fund - Investor Shares  
Average Annual Returns for Periods Ended December 31, 2013  
One Year 0.05%rr_AverageAnnualReturnYear01
/ dei_LegalEntityAxis
= vanguard_S000004462Member
/ rr_PerformanceMeasureAxis
= vanguard_Citigroup3MonthUSTreasuryBillIndexMember
/ rr_ProspectusShareClassAxis
= vanguard_C000012238Member
Five Years 0.09%rr_AverageAnnualReturnYear05
/ dei_LegalEntityAxis
= vanguard_S000004462Member
/ rr_PerformanceMeasureAxis
= vanguard_Citigroup3MonthUSTreasuryBillIndexMember
/ rr_ProspectusShareClassAxis
= vanguard_C000012238Member
Ten Years 1.58%rr_AverageAnnualReturnYear10
/ dei_LegalEntityAxis
= vanguard_S000004462Member
/ rr_PerformanceMeasureAxis
= vanguard_Citigroup3MonthUSTreasuryBillIndexMember
/ rr_ProspectusShareClassAxis
= vanguard_C000012238Member
Citigroup 3-Month U.S. Treasury Bill Index | Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Investor Shares  
Average Annual Returns for Periods Ended December 31, 2013  
One Year 0.05%rr_AverageAnnualReturnYear01
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_PerformanceMeasureAxis
= vanguard_Citigroup3MonthUSTreasuryBillIndexMember
/ rr_ProspectusShareClassAxis
= vanguard_C000012239Member
Five Years 0.09%rr_AverageAnnualReturnYear05
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_PerformanceMeasureAxis
= vanguard_Citigroup3MonthUSTreasuryBillIndexMember
/ rr_ProspectusShareClassAxis
= vanguard_C000012239Member
Ten Years 1.58%rr_AverageAnnualReturnYear10
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_PerformanceMeasureAxis
= vanguard_Citigroup3MonthUSTreasuryBillIndexMember
/ rr_ProspectusShareClassAxis
= vanguard_C000012239Member
Citigroup 3-Month U.S. Treasury Bill Index | Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Institutional Shares  
Average Annual Returns for Periods Ended December 31, 2013  
One Year 0.05%rr_AverageAnnualReturnYear01
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_PerformanceMeasureAxis
= vanguard_Citigroup3MonthUSTreasuryBillIndexMember
/ rr_ProspectusShareClassAxis
= vanguard_C000012240Member
Five Years 0.09%rr_AverageAnnualReturnYear05
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_PerformanceMeasureAxis
= vanguard_Citigroup3MonthUSTreasuryBillIndexMember
/ rr_ProspectusShareClassAxis
= vanguard_C000012240Member
Ten Years 1.58%rr_AverageAnnualReturnYear10
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_PerformanceMeasureAxis
= vanguard_Citigroup3MonthUSTreasuryBillIndexMember
/ rr_ProspectusShareClassAxis
= vanguard_C000012240Member
U.S. Government Money Market Funds Average | Vanguard Federal Money Market Fund | Vanguard Federal Money Market Fund - Investor Shares  
Average Annual Returns for Periods Ended December 31, 2013  
One Year none
Five Years 0.02%rr_AverageAnnualReturnYear05
/ dei_LegalEntityAxis
= vanguard_S000004462Member
/ rr_PerformanceMeasureAxis
= vanguard_USGovernmentMoneyMarketFundsAverageMember
/ rr_ProspectusShareClassAxis
= vanguard_C000012238Member
Ten Years 1.34%rr_AverageAnnualReturnYear10
/ dei_LegalEntityAxis
= vanguard_S000004462Member
/ rr_PerformanceMeasureAxis
= vanguard_USGovernmentMoneyMarketFundsAverageMember
/ rr_ProspectusShareClassAxis
= vanguard_C000012238Member
Money Market Funds Average | Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Investor Shares  
Average Annual Returns for Periods Ended December 31, 2013  
One Year none
Five Years 0.03%rr_AverageAnnualReturnYear05
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_PerformanceMeasureAxis
= vanguard_MoneyMarketFundsAverageMember
/ rr_ProspectusShareClassAxis
= vanguard_C000012239Member
Ten Years 1.37%rr_AverageAnnualReturnYear10
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_PerformanceMeasureAxis
= vanguard_MoneyMarketFundsAverageMember
/ rr_ProspectusShareClassAxis
= vanguard_C000012239Member
Institutional Money Market Funds Average | Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Institutional Shares  
Average Annual Returns for Periods Ended December 31, 2013  
One Year none
Five Years 0.10%rr_AverageAnnualReturnYear05
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_PerformanceMeasureAxis
= vanguard_InstitutionalMoneyMarketFundsAverageMember
/ rr_ProspectusShareClassAxis
= vanguard_C000012240Member
Ten Years 1.63%rr_AverageAnnualReturnYear10
/ dei_LegalEntityAxis
= vanguard_S000004463Member
/ rr_PerformanceMeasureAxis
= vanguard_InstitutionalMoneyMarketFundsAverageMember
/ rr_ProspectusShareClassAxis
= vanguard_C000012240Member