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Class B Prospectus | General Money Market Fund, Inc  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Fund Summary - General Money Market Fund, Inc
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

The fund seeks as high a level of current income as is consistent with the preservation of capital.

Expense [Heading] rr_ExpenseHeading Fees and Expenses
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock

This table describes the fees and expenses that you may pay if you buy and hold shares of the fund.
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock

The Example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the fund for the time periods indicated and then hold or redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the fund's operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategy
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock

To pursue its goal, the fund normally invests in a diversified portfolio of high quality, short-term, dollar-denominated debt securities, including: securities issued or guaranteed as to principal and interest by the U.S. government or its agencies or instrumentalities; certificates of deposit, time deposits, bankers' acceptances and other short-term securities issued by domestic or foreign banks or thrifts or their subsidiaries or branches; repurchase agreements, including tri-party repurchase agreements; asset-backed securities; domestic and dollar-denominated foreign commercial paper; and other short-term corporate obligations, including those with floating or variable rates of interest; and dollar-denominated obligations issued or guaranteed by one or more foreign governments or any of their political subdivisions or agencies. The fund's investments are concentrated in the banking industry, because the fund normally invests at least 25% of its net assets in domestic or dollar-denominated foreign bank obligations.


The fund is a money market fund subject to the maturity, quality, liquidity and diversification requirements of Rule 2a-7 under the Investment Company Act of 1940, as amended, and seeks to maintain a stable share price of $1.00.

Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock

An investment in the fund is not a bank deposit. It is not insured or guaranteed by the Federal Deposit Insurance Corporation (FDIC) or any other government agency. 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's yield will fluctuate as the short-term securities in its portfolio mature or are sold and the proceeds are reinvested in securities with different interest rates. The fund may impose a fee upon the sale of your shares (a "liquidity fee") or may temporarily suspend your ability to sell shares (a redemption "gate") if the fund's liquidity falls below required minimums because of market conditions or other factors. Neither BNY Mellon Investment Adviser, Inc. nor its affiliates have a legal obligation to provide financial support to the fund, and you should not expect that BNY Mellon Investment Adviser, Inc. or its affiliates will provide financial support to the fund at any time. The following are the principal risks that could reduce the fund's income level and/or share price:


· Interest rate risk. This risk refers to the decline in the prices of fixed-income securities that may accompany a rise in the overall level of interest rates. A sharp and unexpected rise in interest rates could impair the fund's ability to maintain a stable net asset value. A low interest rate environment may prevent the fund from providing a positive yield or paying fund expenses out of fund assets and could impair the fund's ability to maintain a stable net asset value.


· Liquidity risk. When there is little or no active trading market for specific types of securities, it can become more difficult to sell the securities in a timely manner at or near their perceived value. In such a market, the value of such securities may fall dramatically, potentially impairing the fund's ability to maintain a stable net asset value.


· Credit risk. Failure of an issuer of a security to make timely interest or principal payments when due, or a decline or perception of a decline in the credit quality of a security, can cause the security's price to fall. Although the fund invests only in high quality debt securities, the credit quality of the securities held by the fund can change rapidly in certain market environments, and the default or a significant price decline of a single holding could impair the fund's ability to maintain a stable net asset value.


· Banking industry risk. The risks generally associated with concentrating investments in the banking industry include interest rate risk, credit risk, and regulatory developments relating to the banking industry.


· Foreign investment risk. The risks generally associated with dollar-denominated foreign investments include economic and political developments, seizure or nationalization of deposits, imposition of taxes or other restrictions on payment of principal and interest.


· U.S. Treasury securities risk. A security backed by the U.S. Treasury or the full faith and credit of the United States is guaranteed only as to the timely payment of interest and principal when held to maturity, but the market prices for such securities are not guaranteed and will fluctuate.


· Government securities risk. Not all obligations of the U.S. government, its agencies and instrumentalities are backed by the full faith and credit of the U.S. Treasury. Some obligations are backed only by the credit of the issuing agency or instrumentality, and in some cases there may be some risk of default by the issuer. Any guarantee by the U.S. government or its agencies or instrumentalities of a security held by the fund does not apply to the market value of such security or to shares of the fund itself. A security backed by the U.S. Treasury or the full faith and credit of the United States is guaranteed only as to the timely payment of interest and principal when held to maturity.


· Repurchase agreement counterparty risk. The fund is subject to the risk that a counterparty in a repurchase agreement and/or, for a tri-party repurchase agreement, the third party bank providing payment administration, collateral custody and management services for the transaction, could fail to honor the terms of the agreement.


· Liquidity fee and/or redemption gate risk. The fund may impose a fee upon the sale of your shares (a "liquidity fee") or may temporarily suspend your ability to sell shares (a redemption "gate") if the fund's liquidity falls below required minimums because of unusual market conditions, an unusually high volume of redemption requests, redemptions by a few large investors, or other reasons. If a liquidity fee is imposed by the fund, it would reduce the amount you will receive upon the redemption of your shares. A "gate" will suspend your ability to redeem your shares while the gate is imposed and may prevent the fund from being able to pay redemption proceeds within the allowable time period stated in this prospectus.


· Market risk. The value of the securities in which the fund invests may be affected by political, regulatory, economic and social developments, and developments that impact specific economic sectors, industries or segments of the market. In addition, turbulence in financial markets and reduced liquidity in equity, credit and/or fixed income markets may negatively affect many issuers, which could adversely affect the fund. Global economies and financial markets are becoming increasingly interconnected, and conditions and events in one country, region or financial market may adversely impact issuers in a different country, region or financial market.These risks may be magnified if certain events or developments adversely interrupt the global supply chain; in these and other circumstances, such risks might affect companies worldwide. Recent examples include pandemic risks related to COVID-19 and aggressive measures taken worldwide in response by governments, including closing borders, restricting international and domestic travel, and the imposition of prolonged quarantines of large populations, and by businesses, including changes to operations and reducing staff. The effects of COVID-19 have contributed to increased volatility in global markets and will likely affect certain countries, companies, industries and market sectors more dramatically than others. The COVID-19 pandemic has had, and any other outbreak of an infectious disease or other serious public health concern could have, a significant negative impact on economic and market conditions and could trigger a prolonged period of global economic slowdown. To the extent the fund may overweight its investments in certain countries, companies, industries or market sectors, such positions will increase the fund's exposure to risk of loss from adverse developments affecting those countries, companies, industries or sectors.
Risk Lose Money [Text] rr_RiskLoseMoney You could lose money by investing in the fund.
Risk Money Market Fund May Impose Fees or Suspend Sales [Text] rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales The fund may impose a fee upon the sale of your shares (a "liquidity fee") or may temporarily suspend your ability to sell shares (a redemption "gate") if the fund's liquidity falls below required minimums because of market conditions or other factors.
Risk Money Market Fund May Not Preserve Dollar [Text] rr_RiskMoneyMarketFundMayNotPreserveDollar Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.
Risk Money Market Fund Price Fluctuates [Text] rr_RiskMoneyMarketFundPriceFluctuates The fund's yield will fluctuate as the short-term securities in its portfolio mature or are sold and the proceeds are reinvested in securities with different interest rates.
Risk Money Market Fund Sponsor May Not Provide Support [Text] rr_RiskMoneyMarketFundSponsorMayNotProvideSupport Neither BNY Mellon Investment Adviser, Inc. nor its affiliates have a legal obligation to provide financial support to the fund, and you should not expect that BNY Mellon Investment Adviser, Inc. or its affiliates will provide financial support to the fund at any time.
RIsk Not Insured [Text] rr_RiskNotInsured An investment in the fund is not a bank deposit. It is not insured or guaranteed by the Federal Deposit Insurance Corporation (FDIC) or any other government agency.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock

The following bar chart and table provide some indication of the risks of investing in the fund. The bar chart shows changes in the performance of the fund's Class B shares from year to year. The table shows the average annual total returns of the fund's Class B shares over time. The fund's past performance is not necessarily an indication of how the fund will perform in the future. More recent performance information may be available at www.dreyfus.com.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The following bar chart and table provide some indication of the risks of investing in the fund.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress www.dreyfus.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The fund's past performance is not necessarily an indication of how the fund will perform in the future.
Bar Chart [Heading] rr_BarChartHeading Year-by-Year Total Returns as of 12/31 each year (%)
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock

Best Quarter
Q1, 2019: 0.41%

Worst Quarter
Q1, 2014: 0.00%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2019
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 0.41%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2014
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn none
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock

For the fund's current yield, call toll free 1-800-373-9387 (inside the U.S. only).
Average Annual Return, Caption rr_AverageAnnualReturnCaption Average Annual Total Returns as of 12/31/19
Money Market Seven Day Yield Phone rr_MoneyMarketSevenDayYieldPhone 1-800-373-9387
Class B Prospectus | General Money Market Fund, Inc | Class B  
Risk/Return: rr_RiskReturnAbstract  
Management fees rr_ManagementFeesOverAssets 0.50%
Distribution (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.20%
Other expenses: Shareholder services fees rr_Component1OtherExpensesOverAssets 0.25%
Other expenses: Sub-accounting services fees rr_Component2OtherExpensesOverAssets 0.05%
Other expenses: Miscellaneous other expenses rr_Component3OtherExpensesOverAssets 0.07%
Total other expenses rr_OtherExpensesOverAssets 0.37%
Total annual fund operating expenses rr_ExpensesOverAssets 1.07%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 109
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 340
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 590
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 1,306
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 109
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 340
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 590
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,306
Annual Return 2010 rr_AnnualReturn2010 0.05%
Annual Return 2011 rr_AnnualReturn2011 0.04%
Annual Return 2012 rr_AnnualReturn2012 0.01%
Annual Return 2013 rr_AnnualReturn2013 0.01%
Annual Return 2014 rr_AnnualReturn2014 0.01%
Annual Return 2015 rr_AnnualReturn2015 0.01%
Annual Return 2016 rr_AnnualReturn2016 0.01%
Annual Return 2017 rr_AnnualReturn2017 0.21%
Annual Return 2018 rr_AnnualReturn2018 1.09%
Annual Return 2019 rr_AnnualReturn2019 1.39%
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 1.39%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 0.54%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 0.28%