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Label Element Value
Risk/Return: rr_RiskReturnAbstract  
Document Type dei_DocumentType 485BPOS
Document Period End Date dei_DocumentPeriodEndDate Aug. 31, 2013
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. 23, 2013
Document Effective Date dei_DocumentEffectiveDate Dec. 23, 2013
Vanguard Federal Money Market Fund | Participant
  
Risk/Return: rr_RiskReturnAbstract  
Prospectus Date rr_ProspectusDate Dec. 23, 2013
Risk/Return rr_RiskReturnHeading Risk/Return
Investment objective: rr_ObjectiveHeading Investment Objective
Investment objective rr_ObjectivePrimaryTextBlock

The Fund seeks to provide current income while maintaining liquidity and a stable share price of $1.
Expenses Heading rr_ExpenseHeading Fees and Expenses
Fees and Expenses of the Fund 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 rr_ShareholderFeesCaption Shareholder Fees (Fees paid directly from your investment)
Operating Expenses Caption rr_OperatingExpensesCaption Annual Fund Operating Expenses (Expenses that you pay each year as a percentage of the value of your investment)
EXAMPLE rr_ExpenseExampleHeading

Example
Expense Example, Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock

The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. It illustrates the hypothetical expenses that you would incur over various periods if you invest $10,000 in the Fund's shares. This example assumes that the Fund provides a return of 5% a year and that total annual fund operating expenses remain as stated in the preceding table. The results apply whether or not you redeem your investment at the end of the given period. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy: rr_StrategyHeading Primary Investment Policies
StrategyNarrative rr_StrategyNarrativeTextBlock

The Fund invests primarily in high-quality, short-term money market instruments. At least 80% of the Fund's assets are invested in securities issued by the U.S. government and its agencies and instrumentalities. Although these securities are high-quality, most of the securities held by the Fund are neither guaranteed by the U.S. Treasury nor supported by the full faith and credit of the U.S. government. To be considered high-quality, a security generally must be rated in one of the two highest credit-quality categories for short-term securities by at least two nationally recognized rating services (or by one, if only one rating service has rated the security). The Fund maintains a dollar-weighted average maturity of 60 days or less and a dollar-weighted average life of 120 days or less.
Risk: rr_RiskHeading Primary Risks
Risk Narrative rr_RiskNarrativeTextBlock

The Fund is designed for investors with a low tolerance for risk; however, the Fund is subject to the following risks, which could affect the Fund's performance:

Income risk, which is the chance that the Fund's income will decline because of falling interest rates. Because the Fund's income is based on short-term interest rates-which can fluctuate significantly over short periods-income risk is expected to be high.

Manager risk, which is the chance that poor security selection will cause the Fund to underperform relevant benchmarks or other funds with a similar investment objective.

Credit risk, which is the chance that the issuer of a security will fail to pay interest and principal in a timely manner or that negative perceptions of the issuer's ability to make such payments will cause the price of that security to decline. Credit risk should be very low for the Fund because it invests primarily in securities that are considered to be of high quality.

An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the Fund seeks to preserve the value of your investment at $1 per share, it is possible to lose money by investing in the Fund.
Money Market Fund rr_RiskMoneyMarketFund

The Fund is designed for investors with a low tolerance for risk; however, the Fund is subject to the following risks, which could affect the Fund's performance:
Not Insured Depository Institution rr_RiskNotInsuredDepositoryInstitution

An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the Fund seeks to preserve the value of your investment at $1 per share, it is possible to lose money by investing in the Fund.
Bar Chart and Performance Table rr_BarChartAndPerformanceTableHeading Annual Total Returns
Performance Information Illustrates Variability of Returns rr_PerformanceInformationIllustratesVariabilityOfReturns

The following bar chart and table are intended to help you understand the risks of investing in the Fund.
Performance Additional Market Index rr_PerformanceAdditionalMarketIndex

, which have investment characteristics similar to those of the Fund.
Performance Availability Phone rr_PerformanceAvailabilityPhone

800-662-7447
Performance Availability Website Address rr_PerformanceAvailabilityWebSiteAddress

vanguard.com/performance

Past Performance Does Not Indiciate Future Returns rr_PerformancePastDoesNotIndicateFuture

Keep in mind that the Fund's past performance does not indicate how the Fund will perform in the future.
Bar Chart rr_BarChartHeading Annual Total Returns - Vanguard Federal Money Market Fund Investor Shares
Bar Chart Narrative rr_BarChartNarrativeTextBlock

The following bar chart and table are intended to help you understand the risks of investing in the Fund. The bar chart shows how the performance of the Fund has varied from one calendar year to another over the periods shown. The table shows how the average annual total returns of the Fund compare with those of a relevant market index and a comparative benchmark, which have investment characteristics similar to those of the Fund. Returns for the U.S. Government Money Market Funds Average are derived from data provided by Lipper, a Thomson Reuters Company. Keep in mind that the Fund's past performance does not indicate how the Fund will perform in the future. Updated performance information is available on our website at vanguard.com/performance or by calling Vanguard toll-free at 800-662-7447.
Bar Chart Footnotes rr_BarChartFootnotesTextBlock

1 The year-to-date return as of the most recent calendar quarter, which ended on September 30, 2013 was 0.02%
Bar Chart Closing rr_BarChartClosingTextBlock

During the periods shown in the bar chart, the highest return for a calendar quarter was 1.28% (quarter ended December 31, 2006 ), and the lowest return for a quarter was 0.00% (quarter ended March 31, 2012 ).
Year to Date Return Label rr_YearToDateReturnLabel The year-to-date return as of the most recent calendar quarter, which ended on September 30, 2013
Highest Quarterly Return Label rr_HighestQuarterlyReturnLabel (quarter ended December 31, 2006 )
Lowest Quarterly Return Label rr_LowestQuarterlyReturnLabel (quarter ended March 31, 2012 )
Bar Chart and Performance Table: rr_PerformanceTableHeading Average Annual Total Returns for Periods Ended December 31, 2012
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees or expenses)
Vanguard Federal Money Market Fund | Retail
  
Risk/Return: rr_RiskReturnAbstract  
Prospectus Date rr_ProspectusDate Dec. 23, 2013
Risk/Return rr_RiskReturnHeading Risk/Return
Investment objective: rr_ObjectiveHeading Investment Objective
Investment objective rr_ObjectivePrimaryTextBlock

The Fund seeks to provide current income while maintaining liquidity and a stable share price of $1.
Expenses Heading rr_ExpenseHeading Fees and Expenses
Fees and Expenses of the Fund 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 rr_ShareholderFeesCaption Shareholder Fees (Fees paid directly from your investment)
Operating Expenses Caption rr_OperatingExpensesCaption Annual Fund Operating Expenses (Expenses that you pay each year as a percentage of the value of your investment)
EXAMPLE rr_ExpenseExampleHeading

Example
Expense Example, Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock

The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. It illustrates the hypothetical expenses that you would incur over various periods if you invest $10,000 in the Fund's shares. This example assumes that the Fund provides a return of 5% a year and that total annual fund operating expenses remain as stated in the preceding table. The results apply whether or not you redeem your investment at the end of the given period. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy: rr_StrategyHeading Primary Investment Policies
StrategyNarrative rr_StrategyNarrativeTextBlock

The Fund invests primarily in high-quality, short-term money market instruments. At least 80% of the Fund's assets are invested in securities issued by the U.S. government and its agencies and instrumentalities. Although these securities are high-quality, most of the securities held by the Fund are neither guaranteed by the U.S. Treasury nor supported by the full faith and credit of the U.S. government. To be considered high-quality, a security generally must be rated in one of the two highest credit-quality categories for short-term securities by at least two nationally recognized rating services (or by one, if only one rating service has rated the security). The Fund maintains a dollar-weighted average maturity of 60 days or less and a dollar-weighted average life of 120 days or less.
Risk: rr_RiskHeading Primary Risks
Risk Narrative rr_RiskNarrativeTextBlock

The Fund is designed for investors with a low tolerance for risk; however, the Fund is subject to the following risks, which could affect the Fund's performance:

Income risk, which is the chance that the Fund's income will decline because of falling interest rates. Because the Fund's income is based on short-term interest rates-which can fluctuate significantly over short periods-income risk is expected to be high.

Manager risk, which is the chance that poor security selection will cause the Fund to underperform relevant benchmarks or other funds with a similar investment objective.

Credit risk, which is the chance that the issuer of a security will fail to pay interest and principal in a timely manner or that negative perceptions of the issuer's ability to make such payments will cause the price of that security to decline. Credit risk should be very low for the Fund because it invests primarily in securities that are considered to be of high quality.

An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the Fund seeks to preserve the value of your investment at $1 per share, it is possible to lose money by investing in the Fund.
Money Market Fund rr_RiskMoneyMarketFund

The Fund is designed for investors with a low tolerance for risk; however, the Fund is subject to the following risks, which could affect the Fund's performance:
Not Insured Depository Institution rr_RiskNotInsuredDepositoryInstitution

An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the Fund seeks to preserve the value of your investment at $1 per share, it is possible to lose money by investing in the Fund.
Bar Chart and Performance Table rr_BarChartAndPerformanceTableHeading Annual Total Returns
Performance Information Illustrates Variability of Returns rr_PerformanceInformationIllustratesVariabilityOfReturns

The following bar chart and table are intended to help you understand the risks of investing in the Fund.
Performance Additional Market Index rr_PerformanceAdditionalMarketIndex

, which have investment characteristics similar to those of the Fund.
Performance Availability Phone rr_PerformanceAvailabilityPhone

800-662-7447
Performance Availability Website Address rr_PerformanceAvailabilityWebSiteAddress

vanguard.com/performance

Past Performance Does Not Indiciate Future Returns rr_PerformancePastDoesNotIndicateFuture

Keep in mind that the Fund's past performance does not indicate how the Fund will perform in the future.
Bar Chart rr_BarChartHeading Annual Total Returns - Vanguard Federal Money Market Fund Investor Shares
Bar Chart Narrative rr_BarChartNarrativeTextBlock

The following bar chart and table are intended to help you understand the risks of investing in the Fund. The bar chart shows how the performance of the Fund has varied from one calendar year to another over the periods shown. The table shows how the average annual total returns of the Fund compare with those of a relevant market index and a comparative benchmark, which have investment characteristics similar to those of the Fund. Returns for the U.S. Government Money Market Funds Average are derived from data provided by Lipper, a Thomson Reuters Company. Keep in mind that the Fund's past performance does not indicate how the Fund will perform in the future. Updated performance information is available on our website at vanguard.com/performance or by calling Vanguard toll-free at 800-662-7447.
Bar Chart Footnotes rr_BarChartFootnotesTextBlock

1 The year-to-date return as of the most recent calendar quarter, which ended on September 30, 2013 was 0.02%
Bar Chart Closing rr_BarChartClosingTextBlock

During the periods shown in the bar chart, the highest return for a calendar quarter was 1.28% (quarter ended December 31, 2006 ), and the lowest return for a quarter was 0.00% (quarter ended March 31, 2012 ).
Year to Date Return Label rr_YearToDateReturnLabel The year-to-date return as of the most recent calendar quarter, which ended on September 30, 2013
Highest Quarterly Return Label rr_HighestQuarterlyReturnLabel (quarter ended December 31, 2006 )
Lowest Quarterly Return Label rr_LowestQuarterlyReturnLabel (quarter ended March 31, 2012 )
Bar Chart and Performance Table: rr_PerformanceTableHeading Average Annual Total Returns for Periods Ended December 31, 2012
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees or expenses)
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 0.12%
12b-1 Distribution Fee rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.02%
Total Annual Fund Operating Expenses rr_NetExpensesOverAssets 0.14% [1]
1 YEAR rr_ExpenseExampleNoRedemptionYear01 14
3 YEAR rr_ExpenseExampleNoRedemptionYear03 45
5 YEAR rr_ExpenseExampleNoRedemptionYear05 79
10 YEAR rr_ExpenseExampleNoRedemptionYear10 179
Annual Return Inception Date rr_AnnualReturnInceptionDate Jul. 13, 1981
2003 rr_AnnualReturn2003 0.89%
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%
Date rr_BarChartYearToDateReturnDate Sep. 30, 2013
Year to Date Return rr_BarChartYearToDateReturn 0.02%
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, 2012
Bar Chart Lowest Quarter Return rr_BarChartLowestQuarterlyReturn 0.00%
One Year rr_AverageAnnualReturnYear01 0.01%
Five Years rr_AverageAnnualReturnYear05 0.59%
Ten Years rr_AverageAnnualReturnYear10 1.76%
Inception Date rr_AverageAnnualReturnInceptionDate Jul. 13, 1981
Vanguard Prime Money Market Fund | Institutional
  
Risk/Return: rr_RiskReturnAbstract  
Prospectus Date rr_ProspectusDate Dec. 23, 2013
Risk/Return rr_RiskReturnHeading Risk/Return
Investment objective: rr_ObjectiveHeading Investment Objective
Investment objective rr_ObjectivePrimaryTextBlock

The Fund seeks to provide current income while maintaining liquidity and a stable share price of $1.
Expenses Heading rr_ExpenseHeading Fees and Expenses
Fees and Expenses of the Fund rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses you may pay if you buy and hold Institutional Shares of the Fund.
Shareholder Fees Caption rr_ShareholderFeesCaption Shareholder Fees (Fees paid directly from your investment)
Operating Expenses Caption rr_OperatingExpensesCaption Annual Fund Operating Expenses (Expenses that you pay each year as a percentage of the value of your investment)
EXAMPLE rr_ExpenseExampleHeading

Example
Expense Example, Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock

The following example is intended to help you compare the cost of investing in the Fund's Institutional Shares with the cost of investing in other mutual funds. It illustrates the hypothetical expenses that you would incur over various periods if you invest $10,000 in the Fund's shares. This example assumes that the Shares provide a return of 5% a year and that total annual fund operating expenses remain as stated in the preceding table. The results apply whether or not you redeem your investment at the end of the given period. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy: rr_StrategyHeading Primary Investment Policies
StrategyNarrative rr_StrategyNarrativeTextBlock

The Fund invests primarily in high-quality, short-term money market instruments, including certificates of deposit, banker's acceptances, commercial paper, and other money market securities. To be considered high-quality, a security generally must be rated in one of the two highest credit-quality categories for short-term securities by at least two nationally recognized rating services (or by one, if only one rating service has rated the security). If unrated, the security must be determined by Vanguard to be of quality equivalent to securities in the two highest credit-quality categories. The Fund invests more than 25% of its assets in securities issued by companies in the financial services industry. The Fund maintains a dollar-weighted average maturity of 60 days or less and a dollar-weighted average life of 120 days or less.
Risk: rr_RiskHeading Primary Risks
Risk Narrative rr_RiskNarrativeTextBlock

The Fund is designed for investors with a low tolerance for risk; however, the Fund is subject to the following risks, which could affect the Fund's performance:

Income risk, which is the chance that the Fund's income will decline because of falling interest rates. Because the Fund's income is based on short-term interest rates-which can fluctuate significantly over short periods-income risk is expected to be high.

Manager risk, which is the chance that poor security selection will cause the Fund to underperform relevant benchmarks or other funds with a similar investment objective.

Credit risk, which is the chance that the issuer of a security will fail to pay interest and principal in a timely manner or that negative perceptions of the issuer's ability to make such payments will cause the price of that security to decline. Credit risk should be very low for the Fund because it invests primarily in securities that are considered to be of high quality.

Industry concentration risk, which is the chance that there will be overall problems affecting a particular industry. Because the Fund invests more than 25% of its assets in securities issued by companies in the financial services industry, the Fund's performance depends to a greater extent on the overall condition of that industry and is more susceptible to events affecting that industry.

An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the Fund seeks to preserve the value of your investment at $1 per share, it is possible to lose money by investing in the Fund.
Money Market Fund rr_RiskMoneyMarketFund

The Fund is designed for investors with a low tolerance for risk; however, the Fund is subject to the following risks, which could affect the Fund's performance:
Not Insured Depository Institution rr_RiskNotInsuredDepositoryInstitution

An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the Fund seeks to preserve the value of your investment at $1 per share, it is possible to lose money by investing in the Fund.
Bar Chart and Performance Table rr_BarChartAndPerformanceTableHeading Annual Total Returns
Performance Information Illustrates Variability of Returns rr_PerformanceInformationIllustratesVariabilityOfReturns

The following bar chart and table are intended to help you understand the risks of investing in the Fund.
Performance Availability Phone rr_PerformanceAvailabilityPhone

800-662-7447
Performance Availability Website Address rr_PerformanceAvailabilityWebSiteAddress

vanguard.com/performance

Past Performance Does Not Indiciate Future Returns rr_PerformancePastDoesNotIndicateFuture

Keep in mind that the Fund's past performance does not indicate how the Fund will perform in the future.
Bar Chart rr_BarChartHeading Annual Total Returns - Vanguard Prime Money Market Fund Institutional Shares
Bar Chart Narrative rr_BarChartNarrativeTextBlock

The following bar chart and table are intended to help you understand the risks of investing in the Fund. The bar chart shows how the performance of the Fund's Institutional Shares has varied from one calendar year to another over the periods shown. The table shows how the average annual total returns of the Institutional Shares compare with those of a relevant market index and a comparative benchmark, which have investment characteristics similar to those of the Fund. Returns for the Institutional Money Market Funds Average are derived from data provided by Lipper Inc. Keep in mind that the Fund's past performance does not indicate how the Fund will perform in the future. Updated performance information is available on our website at vanguard.com/performance or by calling Vanguard toll-free at 800-662-7447.
Bar Chart Footnotes rr_BarChartFootnotesTextBlock

1 The year-to-date return as of the most recent calendar quarter, which ended on September 30, 2013 was 0.05%
Bar Chart Closing rr_BarChartClosingTextBlock

During the periods shown in the bar chart, the highest return for a calendar quarter was 1.34% (quarter ended December 31, 2006 ), and the lowest return for a quarter was 0.03% (quarter ended March 31, 2012 ).
Year to Date Return Label rr_YearToDateReturnLabel The year-to-date return as of the most recent calendar quarter, which ended on September 30, 2013
Highest Quarterly Return Label rr_HighestQuarterlyReturnLabel (quarter ended December 31, 2006 )
Lowest Quarterly Return Label rr_LowestQuarterlyReturnLabel (quarter ended March 31, 2012 )
Bar Chart and Performance Table: rr_PerformanceTableHeading Average Annual Total Returns for Periods Ended December 31, 2012
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees or expenses)
Vanguard Prime Money Market Fund | Participant
  
Risk/Return: rr_RiskReturnAbstract  
Prospectus Date rr_ProspectusDate Dec. 23, 2013
Risk/Return rr_RiskReturnHeading Risk/Return
Investment objective: rr_ObjectiveHeading Investment Objective
Investment objective rr_ObjectivePrimaryTextBlock

The Fund seeks to provide current income while maintaining liquidity and a stable share price of $1.
Expenses Heading rr_ExpenseHeading Fees and Expenses
Fees and Expenses of the Fund rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses you may pay if you buy and hold Investor Shares of the Fund.
Shareholder Fees Caption rr_ShareholderFeesCaption Shareholder Fees (Fees paid directly from your investment)
Operating Expenses Caption rr_OperatingExpensesCaption Annual Fund Operating Expenses (Expenses that you pay each year as a percentage of the value of your investment)
EXAMPLE rr_ExpenseExampleHeading

Example
Expense Example, Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock

The following example is intended to help you compare the cost of investing in the Fund's Investor Shares with the cost of investing in other mutual funds. It illustrates the hypothetical expenses that you would incur over various periods if you invest $10,000 in the Fund's shares. This example assumes that the Shares provide a return of 5% a year and that total annual fund operating expenses remain as stated in the preceding table. The results apply whether or not you redeem your investment at the end of the given period. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy: rr_StrategyHeading Primary Investment Policies
StrategyNarrative rr_StrategyNarrativeTextBlock

The Fund invests primarily in high-quality, short-term money market instruments, including certificates of deposit, banker's acceptances, commercial paper, and other money market securities. To be considered high-quality, a security generally must be rated in one of the two highest credit-quality categories for short-term securities by at least two nationally recognized rating services (or by one, if only one rating service has rated the security). If unrated, the security must be determined by Vanguard to be of quality equivalent to securities in the two highest credit-quality categories. The Fund invests more than 25% of its assets in securities issued by companies in the financial services industry. The Fund maintains a dollar-weighted average maturity of 60 days or less and a dollar-weighted average life of 120 days or less.
Risk: rr_RiskHeading Primary Risks
Risk Narrative rr_RiskNarrativeTextBlock

The Fund is designed for investors with a low tolerance for risk; however, the Fund is subject to the following risks, which could affect the Fund's performance:

Income risk, which is the chance that the Fund's income will decline because of falling interest rates. Because the Fund's income is based on short-term interest rates-which can fluctuate significantly over short periods-income risk is expected to be high.

Manager risk, which is the chance that poor security selection will cause the Fund to underperform relevant benchmarks or other funds with a similar investment objective.

Credit risk, which is the chance that the issuer of a security will fail to pay interest and principal in a timely manner or that negative perceptions of the issuer's ability to make such payments will cause the price of that security to decline. Credit risk should be very low for the Fund because it invests primarily in securities that are considered to be of high quality.

Industry concentration risk, which is the chance that there will be overall problems affecting a particular industry. Because the Fund invests more than 25% of its assets in securities issued by companies in the financial services industry, the Fund's performance depends to a greater extent on the overall condition of that industry and is more susceptible to events affecting that industry.

An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the Fund seeks to preserve the value of your investment at $1 per share, it is possible to lose money by investing in the Fund.
May Lose Money rr_RiskLoseMoney

The Fund is subject to the risks associated with the stock and bond markets, any of which could cause an investor to lose money.
Money Market Fund rr_RiskMoneyMarketFund

The Fund is designed for investors with a low tolerance for risk; however, the Fund is subject to the following risks, which could affect the Fund's performance:
Not Insured Depository Institution rr_RiskNotInsuredDepositoryInstitution

An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the Fund seeks to preserve the value of your investment at $1 per share, it is possible to lose money by investing in the Fund.
Bar Chart and Performance Table rr_BarChartAndPerformanceTableHeading Annual Total Returns
Performance Information Illustrates Variability of Returns rr_PerformanceInformationIllustratesVariabilityOfReturns

The following bar chart and table are intended to help you understand the risks of investing in the Fund.
Performance Additional Market Index rr_PerformanceAdditionalMarketIndex

, which have investment characteristics similar to those of the Fund.
Performance Availability Phone rr_PerformanceAvailabilityPhone

800-662-7447
Performance Availability Website Address rr_PerformanceAvailabilityWebSiteAddress

vanguard.com/performance

Past Performance Does Not Indiciate Future Returns rr_PerformancePastDoesNotIndicateFuture

Keep in mind that the Fund's past performance does not indicate how the Fund will perform in the future.
Bar Chart rr_BarChartHeading Annual Total Returns - Vanguard Prime Money Market Fund Investor Shares
Bar Chart Narrative rr_BarChartNarrativeTextBlock

The following bar chart and table are intended to help you understand the risks of investing in the Fund. The bar chart shows how the performance of the Fund's Investor Shares has varied from one calendar year to another over the periods shown. The table shows how the average annual total returns of the Investor Shares compare with those of a relevant market index and a comparative benchmark, which have investment characteristics similar to those of the Fund. Returns for the Money Market Funds Average are derived from data provided by Lipper Inc. Keep in mind that the Fund's past performance does not indicate how the Fund will perform in the future. Updated performance information is available on our website at vanguard.com/performance or by calling Vanguard toll-free at 800-662-7447.
Bar Chart Footnotes rr_BarChartFootnotesTextBlock

1 The year-to-date return as of the most recent calendar quarter, which ended on September 30, 2013 was 0.02%
Bar Chart Closing rr_BarChartClosingTextBlock

During the periods shown in the bar chart, the highest return for a calendar quarter was 1.29% (quarter ended December 31, 2006 ), and the lowest return for a quarter was 0.01% (quarter ended March 31, 2010 ).
Year to Date Return Label rr_YearToDateReturnLabel The year-to-date return as of the most recent calendar quarter, which ended on September 30, 2013
Highest Quarterly Return Label rr_HighestQuarterlyReturnLabel (quarter ended December 31, 2006 )
Lowest Quarterly Return Label rr_LowestQuarterlyReturnLabel (quarter ended March 31, 2010 )
Bar Chart and Performance Table: rr_PerformanceTableHeading Average Annual Total Returns for Periods Ended December 31, 2012
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees or expenses)
Vanguard Prime Money Market Fund | Retail
  
Risk/Return: rr_RiskReturnAbstract  
Prospectus Date rr_ProspectusDate Dec. 28, 2012
Risk/Return rr_RiskReturnHeading Risk/Return
Investment objective: rr_ObjectiveHeading Investment Objective
Investment objective rr_ObjectivePrimaryTextBlock

The Fund seeks to provide current income while maintaining liquidity and a stable share price of $1.
Expenses Heading rr_ExpenseHeading Fees and Expenses
Fees and Expenses of the Fund rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses you may pay if you buy and hold Investor Shares of the Fund.
Shareholder Fees Caption rr_ShareholderFeesCaption Shareholder Fees (Fees paid directly from your investment)
Operating Expenses Caption rr_OperatingExpensesCaption Annual Fund Operating Expenses (Expenses that you pay each year as a percentage of the value of your investment)
EXAMPLE rr_ExpenseExampleHeading

Example
Expense Example, Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock

The following example is intended to help you compare the cost of investing in the Fund's Investor Shares with the cost of investing in other mutual funds. It illustrates the hypothetical expenses that you would incur over various periods if you invest $10,000 in the Fund's shares. This example assumes that the Shares provide a return of 5% a year and that total annual fund operating expenses remain as stated in the preceding table. The results apply whether or not you redeem your investment at the end of the given period. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy: rr_StrategyHeading Primary Investment Policies
StrategyNarrative rr_StrategyNarrativeTextBlock

The Fund invests primarily in high-quality, short-term money market instruments, including certificates of deposit, banker's acceptances, commercial paper, and other money market securities. To be considered high-quality, a security generally must be rated in one of the two highest credit-quality categories for short-term securities by at least two nationally recognized rating services (or by one, if only one rating service has rated the security). If unrated, the security must be determined by Vanguard to be of quality equivalent to securities in the two highest credit-quality categories. The Fund invests more than 25% of its assets in securities issued by companies in the financial services industry. The Fund maintains a dollar-weighted average maturity of 60 days or less and a dollar-weighted average life of 120 days or less.
Risk: rr_RiskHeading Primary Risks
Risk Narrative rr_RiskNarrativeTextBlock

The Fund is designed for investors with a low tolerance for risk; however, the Fund is subject to the following risks, which could affect the Fund's performance:

Income risk, which is the chance that the Fund's income will decline because of falling interest rates. Because the Fund's income is based on short-term interest rates-which can fluctuate significantly over short periods-income risk is expected to be high.

Manager risk, which is the chance that poor security selection will cause the Fund to underperform relevant benchmarks or other funds with a similar investment objective.

Credit risk, which is the chance that the issuer of a security will fail to pay interest and principal in a timely manner or that negative perceptions of the issuer's ability to make such payments will cause the price of that security to decline. Credit risk should be very low for the Fund because it invests primarily in securities that are considered to be of high quality.

Industry concentration risk, which is the chance that there will be overall problems affecting a particular industry. Because the Fund invests more than 25% of its assets in securities issued by companies in the financial services industry, the Fund's performance depends to a greater extent on the overall condition of that industry and is more susceptible to events affecting that industry.

An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the Fund seeks to preserve the value of your investment at $1 per share, it is possible to lose money by investing in the Fund.
Money Market Fund rr_RiskMoneyMarketFund

The Fund is designed for investors with a low tolerance for risk; however, the Fund is subject to the following risks, which could affect the Fund's performance:
Not Insured Depository Institution rr_RiskNotInsuredDepositoryInstitution

An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the Fund seeks to preserve the value of your investment at $1 per share, it is possible to lose money by investing in the Fund.
Bar Chart and Performance Table rr_BarChartAndPerformanceTableHeading Annual Total Returns
Performance Information Illustrates Variability of Returns rr_PerformanceInformationIllustratesVariabilityOfReturns

The following bar chart and table are intended to help you understand the risks of investing in the Fund.
Performance Additional Market Index rr_PerformanceAdditionalMarketIndex

, which have investment characteristics similar to those of the Fund.
Performance Availability Phone rr_PerformanceAvailabilityPhone

800-662-7447
Performance Availability Website Address rr_PerformanceAvailabilityWebSiteAddress

vanguard.com/performance

Past Performance Does Not Indiciate Future Returns rr_PerformancePastDoesNotIndicateFuture

Keep in mind that the Fund's past performance does not indicate how the Fund will perform in the future.
Bar Chart rr_BarChartHeading Annual Total Returns - Vanguard Prime Money Market Fund Investor Shares
Bar Chart Narrative rr_BarChartNarrativeTextBlock

The following bar chart and table are intended to help you understand the risks of investing in the Fund. The bar chart shows how the performance of the Fund's Investor Shares has varied from one calendar year to another over the periods shown. The table shows how the average annual total returns of the Investor Shares compare with those of a relevant market index and a comparative benchmark, which have investment characteristics similar to those of the Fund. Returns for the Money Market Funds Average are derived from data provided by Lipper Inc. Keep in mind that the Fund's past performance does not indicate how the Fund will perform in the future. Updated performance information is available on our website at vanguard.com/performance or by calling Vanguard toll-free at 800-662-7447.
Bar Chart Footnotes rr_BarChartFootnotesTextBlock

1 The year-to-date return as of the most recent calendar quarter, which ended on September 30, 2013 was 0.02%
Bar Chart Closing rr_BarChartClosingTextBlock

During the periods shown in the bar chart, the highest return for a calendar quarter was 1.29% (quarter ended December 31, 2006 ), and the lowest return for a quarter was 0.01% (quarter ended March 31, 2010 ).
Year to Date Return Label rr_YearToDateReturnLabel The year-to-date return as of the most recent calendar quarter, which ended on September 30, 2013
Highest Quarterly Return Label rr_HighestQuarterlyReturnLabel (quarter ended December 31, 2006 )
Lowest Quarterly Return Label rr_LowestQuarterlyReturnLabel (quarter ended March 31, 2010 )
Bar Chart and Performance Table: rr_PerformanceTableHeading Average Annual Total Returns for Periods Ended December 31, 2012
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees or expenses)
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 0.14%
12b-1 Distribution Fee rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.03%
Total Annual Fund Operating Expenses rr_NetExpensesOverAssets 0.17% [1]
1 YEAR rr_ExpenseExampleNoRedemptionYear01 17
3 YEAR rr_ExpenseExampleNoRedemptionYear03 55
5 YEAR rr_ExpenseExampleNoRedemptionYear05 96
10 YEAR rr_ExpenseExampleNoRedemptionYear10 217
Annual Return Inception Date rr_AnnualReturnInceptionDate Jun. 04, 1975
2003 rr_AnnualReturn2003 0.90%
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%
Date rr_BarChartYearToDateReturnDate Sep. 30, 2013
Year to Date Return rr_BarChartYearToDateReturn 0.02%
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, 2010
Bar Chart Lowest Quarter Return rr_BarChartLowestQuarterlyReturn 0.01%
One Year rr_AverageAnnualReturnYear01 0.04%
Five Years rr_AverageAnnualReturnYear05 0.68%
Ten Years rr_AverageAnnualReturnYear10 1.83%
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
2003 rr_AnnualReturn2003 1.11%
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%
Date rr_BarChartYearToDateReturnDate Sep. 30, 2013
Year to Date Return rr_BarChartYearToDateReturn 0.05%
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 Mar. 31, 2012
Bar Chart Lowest Quarter Return rr_BarChartLowestQuarterlyReturn 0.03%
One Year rr_AverageAnnualReturnYear01 0.11%
Five Years rr_AverageAnnualReturnYear05 0.80%
Ten Years rr_AverageAnnualReturnYear10 1.99%
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 0.07%
Five Years rr_AverageAnnualReturnYear05 0.44%
Ten Years rr_AverageAnnualReturnYear10 1.69%
Citigroup 3-Month U.S. Treasury Bill Index | Vanguard Federal Money Market Fund | Vanguard Federal Money Market Fund - Investor Shares | Participant
  
Risk/Return: rr_RiskReturnAbstract  
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees or expenses)
Citigroup 3-Month U.S. Treasury Bill Index | Vanguard Federal Money Market Fund | Vanguard Federal Money Market Fund - Investor Shares | Retail
  
Risk/Return: rr_RiskReturnAbstract  
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees or expenses)
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 0.07%
Five Years rr_AverageAnnualReturnYear05 0.44%
Ten Years rr_AverageAnnualReturnYear10 1.69%
Citigroup 3-Month U.S. Treasury Bill Index | Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Investor Shares | Participant
  
Risk/Return: rr_RiskReturnAbstract  
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees or expenses)
Citigroup 3-Month U.S. Treasury Bill Index | Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Investor Shares | Retail
  
Risk/Return: rr_RiskReturnAbstract  
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees or expenses)
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 0.07%
Five Years rr_AverageAnnualReturnYear05 0.44%
Ten Years rr_AverageAnnualReturnYear10 1.69%
Citigroup 3-Month U.S. Treasury Bill Index | Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Institutional Shares | Institutional
  
Risk/Return: rr_RiskReturnAbstract  
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees or expenses)
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 0.35%
Ten Years rr_AverageAnnualReturnYear10 1.38%
U.S. Government Money Market Funds Average | Vanguard Federal Money Market Fund | Vanguard Federal Money Market Fund - Investor Shares | Participant
  
Risk/Return: rr_RiskReturnAbstract  
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes   
U.S. Government Money Market Funds Average | Vanguard Federal Money Market Fund | Vanguard Federal Money Market Fund - Investor Shares | Retail
  
Risk/Return: rr_RiskReturnAbstract  
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes   
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 0.44%
Ten Years rr_AverageAnnualReturnYear10 1.42%
Money Market Funds Average | Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Investor Shares | Participant
  
Risk/Return: rr_RiskReturnAbstract  
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes   
Money Market Funds Average | Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Investor Shares | Retail
  
Risk/Return: rr_RiskReturnAbstract  
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes   
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 0.06%
Five Years rr_AverageAnnualReturnYear05 0.59%
Ten Years rr_AverageAnnualReturnYear10 1.71%
Institutional Money Market Funds Average | Vanguard Prime Money Market Fund | Vanguard Prime Money Market Fund - Institutional Shares | Institutional
  
Risk/Return: rr_RiskReturnAbstract  
Index No Deduction for Fees, Expenses, Taxes rr_IndexNoDeductionForFeesExpensesTaxes   
[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.