| Label | Element | Value |
|---|---|---|
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Registrant Name | dei_EntityRegistrantName | VANGUARD CMT FUNDS |
| Prospectus Date | rr_ProspectusDate | Dec. 20, 2024 |
| Investor | Vanguard Market Liquidity Fund | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Risk/Return [Heading] | rr_RiskReturnHeading | <span style="color:#000000;font-family:Arial;font-size:13pt;font-weight:bold;">Fund Summary</span> |
| Objective [Heading] | rr_ObjectiveHeading | <span style="color:#000000;font-family:Arial;font-size:10pt;font-weight:bold;">Investment Objective</span> |
| Objective, Primary [Text Block] | rr_ObjectivePrimaryTextBlock | The Fund seeks to provide current income while maintaining liquidity. |
| Expense [Heading] | rr_ExpenseHeading | <span style="color:#000000;font-family:Arial;font-size:10pt;font-weight:bold;">Fees and Expenses</span> |
| Expense Narrative [Text Block] | rr_ExpenseNarrativeTextBlock | The following tables describe 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 tables and example below. |
| Shareholder Fees Caption [Text] | rr_ShareholderFeesCaption | <span style="color:#000000;font-family:Arial;font-size:8.5pt;font-weight:bold;">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;">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;">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;">Principal Investment Strategies</span> |
| Strategy Narrative [Text Block] | rr_StrategyNarrativeTextBlock | The Fund invests in primarily 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 or 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 maintains a dollar-weighted average maturity of less than 60 days 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. The Fund will generally invest 100% of its assets in U.S. government securities (including repurchase agreements) and cash, 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;">Principal Risks</span> |
| Risk [Text Block] | rr_RiskTextBlock | 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, and the level of risk may vary based on market conditions:• 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.• 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 high quality.• 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.• Repurchase agreements risk, which is the chance that a counterparty to a repurchase agreement may not fulfill its contractual obligations causing the Fund to lose money, suffer delays, or incur costs arising from holding or selling the underlying security.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.The Fund may impose a fee upon sale of your shares. An investment in the Fund is not a bank account and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. The Fund’s sponsor is not required to reimburse the Fund, and you should not expect that the sponsor will provide financial support to the Fund at any time, including during periods of market stress. |
| Bar Chart and Performance Table [Heading] | rr_BarChartAndPerformanceTableHeading | <span style="color:#000000;font-family:Arial;font-size:10pt;font-weight:bold;">Annual Total Returns</span> |
| Performance Narrative [Text Block] | rr_PerformanceNarrativeTextBlock | 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 comparative benchmark, which has 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. The Fund is using a benchmark that is different from the one used for the immediately preceding fiscal year to reflect that the Fund is currently a government money market fund, whereas in the preceding fiscal year, the Fund had been an institutional prime money market fund. Information regarding the former benchmark, the Institutional Money Market Funds Average, derived from data provided by Lipper, a Thomson Reuters Company, is also provided. Prior to June 5, 2024, the Fund operated as an institutional prime money market fund and invested in certain types of securities that the Fund is no longer permitted to hold. Consequently, the performance information below may have been different if the current investment limitation had been in effect during the period prior to the Fund’s conversion to a government money market fund.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. |
| Performance Information Illustrates Variability of Returns [Text] | rr_PerformanceInformationIllustratesVariabilityOfReturns | <span style="font-family:Arial;font-size:9.5pt;">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 comparative benchmark, which has investment characteristics similar to those of the Fund.</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 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;">Annual Total Returns — Vanguard Market Liquidity Fund Investor 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;">Average Annual Total Returns for Periods Ended December 31, 2023</span> |
| Investor | Vanguard Market Liquidity Fund | Risk Lose Money [Member] | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Risk [Text Block] | rr_RiskTextBlock | You could lose money by investing in the Fund. |
| Investor | Vanguard Market Liquidity Fund | Risk Money Market Fund Price Fluctuates [Member] | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Risk [Text Block] | rr_RiskTextBlock | Although the Fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. |
| Investor | Vanguard Market Liquidity Fund | Risk Money Market Fund May Impose Fees or Suspend Sales [Member] | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Risk [Text Block] | rr_RiskTextBlock | The Fund may impose a fee upon sale of your shares. |
| Investor | Vanguard Market Liquidity Fund | Risk Money Market Fund Sponsor May Not Provide Support [Member] | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Risk [Text Block] | rr_RiskTextBlock | The Fund’s sponsor is not required to reimburse the Fund, and you should not expect that the sponsor will provide financial support to the Fund at any time, including during periods of market stress. |
| Investor | Vanguard Market Liquidity Fund | Risk Not Insured [Member] | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Risk [Text Block] | rr_RiskTextBlock | An investment in the Fund is not a bank account and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. |
| Investor | Vanguard Market Liquidity Fund | Investor 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 |
| Management Fees | rr_ManagementFeesOverAssets | 0.005% |
| 12b-1 Distribution Fee | rr_DistributionAndService12b1FeesOverAssets | none |
| Other Expenses | rr_OtherExpensesOverAssets | none |
| Total Annual Fund Operating Expenses | rr_ExpensesOverAssets | 0.005% |
| 1 Year | rr_ExpenseExampleYear01 | $ 1 |
| 3 Years | rr_ExpenseExampleYear03 | 2 |
| 5 Years | rr_ExpenseExampleYear05 | 3 |
| 10 Years | rr_ExpenseExampleYear10 | $ 6 |
| Year to Date Return, Label | rr_YearToDateReturnLabel | year-to-date return |
| Bar Chart, Year to Date Return, Date | rr_BarChartYearToDateReturnDate | Sep. 30, 2024 |
| Bar Chart, Year to Date Return | rr_BarChartYearToDateReturn | 4.11% |
| Highest Quarterly Return, Label | rr_HighestQuarterlyReturnLabel | <span style="font-family:Arial;font-size:8pt;margin-left:0.0pt;">Highest</span> |
| Highest Quarterly Return, Date | rr_BarChartHighestQuarterlyReturnDate | Dec. 31, 2023 |
| Highest Quarterly Return | rr_BarChartHighestQuarterlyReturn | 1.37% |
| Lowest Quarterly Return, Label | rr_LowestQuarterlyReturnLabel | <span style="font-family:Arial;font-size:8pt;margin-left:0.0pt;">Lowest</span> |
| Lowest Quarterly Return, Date | rr_BarChartLowestQuarterlyReturnDate | Dec. 31, 2021 |
| Lowest Quarterly Return | rr_BarChartLowestQuarterlyReturn | 0.01% |
| 1 Year | rr_AverageAnnualReturnYear01 | 5.19% |
| 5 Years | rr_AverageAnnualReturnYear05 | 1.96% |
| 10 Years | rr_AverageAnnualReturnYear10 | 1.38% |
| Investor | Vanguard Market Liquidity Fund | Institutional Money Market Funds Average | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| 1 Year | rr_AverageAnnualReturnYear01 | 4.84% |
| 5 Years | rr_AverageAnnualReturnYear05 | 1.76% |
| 10 Years | rr_AverageAnnualReturnYear10 | 1.17% |
| Investor | Vanguard Market Liquidity Fund | U.S. Government Money Market Funds Average | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| 1 Year | rr_AverageAnnualReturnYear01 | 4.60% |
| 5 Years | rr_AverageAnnualReturnYear05 | 1.53% |
| 10 Years | rr_AverageAnnualReturnYear10 | 0.92% |