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Risk Return Abstract rr_RiskReturnAbstract  
Prospectus Date rr_ProspectusDate Dec. 17, 2021
Retail | Vanguard Cash Reserves Federal Money Market Fund  
Risk Return Abstract rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading <span style="color:#000000;font-family:Arial;font-size:13pt;font-weight:bold;margin-left:0%;">Vanguard Cash Reserves Federal Money Market Fund</span>
Objective [Heading] rr_ObjectiveHeading <span style="color:#000000;font-family:Arial;font-size:10pt;font-weight:bold;margin-left:0%;">Investment Objective</span>
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The Fund seeks to provide current income while maintaining liquidity and a stable share price of $1.
Expense [Heading] rr_ExpenseHeading <span style="color:#000000;font-family:Arial;font-size:10pt;font-weight:bold;margin-left:0%;">Fees and Expenses</span>
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock The following table describes the fees and expenses you may pay if you buy, hold, and sell shares of the Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the table and example below.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption <span style="color:#000000;font-family:Arial;font-size:8.5pt;font-weight:bold;margin-left:0%;">Shareholder Fees</span> <br/><span style="color:#000000;font-family:Arial;font-size:8.5pt;">(Fees paid directly from your investment)</span>
Operating Expenses Caption [Text] rr_OperatingExpensesCaption <span style="color:#000000;font-family:Arial;font-size:8.5pt;font-weight:bold;margin-left:0%;">Annual Fund Operating Expenses</span> <br/><span style="color:#000000;font-family:Arial;font-size:8.5pt;">(Expenses that you pay each year as a percentage of the value of your investment)</span>
Expense Example [Heading] rr_ExpenseExampleHeading <span style="color:#000000;font-family:Arial;font-size:9.5pt;margin-left:0%;">Example</span>
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 were to invest $10,000 in the Fund's shares. This example assumes that the Fund provides a return of 5% each year and that total annual fund operating expenses remain as stated in the preceding table. You would incur these hypothetical expenses whether or not you were to 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 [Heading] rr_StrategyHeading <span style="color:#000000;font-family:Arial;font-size:10pt;font-weight:bold;margin-left:0%;">Principal Investment Strategies</span>
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock The Fund invests primarily in high-quality, short-term money market instruments. Under normal circumstances, at least 80% of the Fund’s assets are invested in securities issued by the U.S. government and its agencies and instrumentalities, including repurchase agreements that are collateralized solely by U.S. government securities or cash. Although these securities are high-quality, some 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 must be determined by Vanguard to present minimal credit risk based in part on a consideration of maturity, portfolio diversification, portfolio liquidity, and credit quality. The Fund invests more than 25% of its assets in securities issued by companies in the financial services industry, which includes, without limitation, securities issued by certain government-sponsored enterprises. The Fund maintains a dollar-weighted average maturity of 60 days or less and a dollar-weighted average life of 120 days or less.Government money market funds are required to invest at least 99.5% of their total assets in cash, U.S. government securities, and/or repurchase agreements that are collateralized solely by U.S. government securities or cash (collectively, government securities). The Fund generally invests 100% of its assets in U.S. government securities and therefore will satisfy the 99.5% requirement for designation as a government money market fund.
Risk [Heading] rr_RiskHeading <span style="color:#000000;font-family:Arial;font-size:10pt;font-weight:bold;margin-left:0%;">Principal Risks</span>
Risk Narrative [Text Block] 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. A low or negative interest rate environment will adversely affect the Fund’s return. Low or negative interest rates, depending on their duration and severity, could prevent the Fund from, among other things, providing a positive yield and/or maintaining a stable share price of $1.•  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 or 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.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. 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 <span style="color:#000000;font-family:Arial;font-size:9.5pt;font-weight:bold;margin-left:0%;">You could lose money by investing in the Fund.</span>
Risk Money Market Fund May Not Preserve Dollar [Text] rr_RiskMoneyMarketFundMayNotPreserveDollar <span style="color:#000000;font-family:Arial;font-size:9.5pt;font-weight:bold;"> Although the Fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. </span>
Risk Not Insured [Text] rr_RiskNotInsured <span style="color:#000000;font-family:Arial;font-size:9.5pt;font-weight:bold;">An investment in the Fund is not insured or </span><span style="color:#000000;font-family:Arial;font-size:9.5pt;font-weight:bold;">guaranteed by the Federal Deposit Insurance Corporation or any other government agency. </span>
Risk Money Market Fund Sponsor May Not Provide Support [Text] rr_RiskMoneyMarketFundSponsorMayNotProvideSupport <span style="color:#000000;font-family:Arial;font-size:9.5pt;font-weight:bold;">The Fund's sponsor has no legal obligation to provide </span><span style="color:#000000;font-family:Arial;font-size:9.5pt;font-weight:bold;">financial support to the Fund, and you should not expect that the sponsor will provide financial support to the Fund at any time.</span>
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading <span style="color:#000000;font-family:Arial;font-size:10pt;font-weight:bold;margin-left:0%;">Annual Total Returns</span>
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock Effective September 29, 2020, the Fund changed its investment strategy and name, and changed its designation to a “government” money market fund. Performance for the periods prior to September 29, 2020, is based on the investment strategy utilized by the Fund prior to September 29, 2020, under the name Vanguard Prime Money Market Fund. 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.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns <span style="font-family:Arial;font-size:9.5pt;">The following</span><span style="font-family:Arial;font-size:9.5pt;"> 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.</span>
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone <span style="font-family:Arial;font-size:9.5pt;">800-662-7447</span>
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress <span style="font-family:Arial;font-size:9.5pt;font-style:italic;">vanguard.com/performance</span>
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture <span style="font-family:Arial;font-size:9.5pt;">Keep in </span><span style="font-family:Arial;font-size:9.5pt;">mind that the Fund's past performance does not indicate how the Fund will perform in the future.</span>
Bar Chart [Heading] rr_BarChartHeading <span style="font-family:Arial;font-size:8pt;font-weight:bold;margin-left:0%;">Annual Total Returns — Vanguard Cash Reserves Federal Money Market Fund Admiral </span><span style="font-family:Arial;font-size:8pt;font-weight:bold;">Shares</span><span style="font-family:Arial;font-size:5pt;font-weight:bold;position:relative;top:-3.25pt;">1</span>
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock During the periods shown in the bar chart, the highest and lowest returns for a calendar quarter were:
Performance Table Heading rr_PerformanceTableHeading <span style="font-family:Arial;font-size:8.5pt;font-weight:bold;margin-left:0%;">Average Annual Total Returns for Periods Ended December 31, </span><span style="font-family:Arial;font-size:8.5pt;font-weight:bold;">2020</span>
Retail | Vanguard Cash Reserves Federal Money Market Fund | Admiral Shares  
Risk Return Abstract rr_RiskReturnAbstract  
Purchase Fee rr_MaximumCumulativeSalesChargeOverOther none
Sales Charge (Load) Imposed on Purchases rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Sales Charge (Load) Imposed on Reinvested Dividends rr_MaximumSalesChargeOnReinvestedDividendsAndDistributionsOverOther none
Redemption Fee rr_RedemptionFeeOverRedemption none
Account Service Fee Per Year (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 0.01%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.10% [1]
1 Year rr_ExpenseExampleYear01 $ 10
3 Years rr_ExpenseExampleYear03 32
5 Years rr_ExpenseExampleYear05 56
10 Years rr_ExpenseExampleYear10 $ 128
2011 rr_AnnualReturn2011 0.14%
2012 rr_AnnualReturn2012 0.11%
2013 rr_AnnualReturn2013 0.06%
2014 rr_AnnualReturn2014 0.05%
2015 rr_AnnualReturn2015 0.11%
2016 rr_AnnualReturn2016 0.55%
2017 rr_AnnualReturn2017 1.08%
2018 rr_AnnualReturn2018 2.01%
2019 rr_AnnualReturn2019 2.29%
2020 rr_AnnualReturn2020 0.57%
Year to Date Return, Label rr_YearToDateReturnLabel year-to-date return
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Sep. 30, 2021
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 0.01%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel <span style="font-family:Arial;font-size:8pt;padding-left:0.0%;">Highest</span>
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2019
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 0.62%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel <span style="font-family:Arial;font-size:8pt;padding-left:0.0%;">Lowest</span>
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2020
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn 0.01%
1 Year rr_AverageAnnualReturnYear01 0.57%
5 Years rr_AverageAnnualReturnYear05 1.30%
10 Years rr_AverageAnnualReturnYear10 0.70%
Retail | Vanguard Cash Reserves Federal Money Market Fund | FTSE 3-Month U.S. Treasury Bill Index (reflects no deduction for fees or expenses)  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 0.58%
5 Years rr_AverageAnnualReturnYear05 1.16%
10 Years rr_AverageAnnualReturnYear10 0.60%
Retail | Vanguard Cash Reserves Federal Money Market Fund | U.S. Government Money Market Funds Average  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 0.24%
5 Years rr_AverageAnnualReturnYear05 0.69%
10 Years rr_AverageAnnualReturnYear10 0.34%
[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.