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BNY Mellon Stock Index Fund, Inc.  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Fund Summary
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

The fund seeks to match the total return of the S&P 500® Index.
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. These figures do not reflect any fees or charges imposed by participating insurance companies under their Variable Annuity contracts (VA contracts) or Variable Life Insurance policies (VLI policies), and if such fees and/or charges were included, the fees and expenses would be higher.
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)
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading Portfolio Turnover
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock

The fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover may indicate higher transaction costs. These costs, which are not reflected in annual fund operating expenses or in the Example, affect the fund's performance. During the most recent fiscal year, the fund's portfolio turnover rate was 2.94% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 2.94%
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. The Example does not reflect fees and expenses incurred under VA contracts and VLI policies; if they were reflected, the figures in the Example would be higher. 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 generally is fully invested in stocks included in the S&P 500® Index. The fund generally invests in all 500 stocks in the S&P 500 Index in proportion to their weighting in the index.

The S&P 500 Index is an unmanaged index of 500 common stocks chosen to reflect the industries of the U.S. economy and is often considered a proxy for the stock market in general. S&P weights each company's stock in the index by its market capitalization (i.e., the share price times the number of shares outstanding), adjusted by the number of available float shares (i.e., those shares available to public investors). Companies included in the S&P 500 Index generally must have market capitalizations in excess of $8.2 billion, to the extent consistent with market conditions.



"Standard & Poor's®," "S&P®," "Standard & Poor's® 500" and "S&P 500®" are trademarks of Standard & Poor's Financial Services LLC (Standard & Poor's) and have been licensed for use by the fund. The fund is not sponsored, endorsed, sold or promoted by Standard & Poor's and Standard & Poor's makes no representation regarding the advisability of investing in the fund.
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. It is not a complete investment program. The fund's share price fluctuates, sometimes dramatically, which means you could lose money.


· Risks of stock investing. Stocks generally fluctuate more in value than bonds and may decline significantly over short time periods. There is the chance that stock prices overall will decline because stock markets tend to move in cycles, with periods of rising prices and falling prices. The market value of a stock may decline due to general market conditions or because of factors that affect the particular company or the company's industry.


· Indexing strategy risk. The fund uses an indexing strategy. It does not attempt to manage market volatility, use defensive strategies or reduce the effects of any long-term periods of poor index performance. The correlation between fund and index performance may be affected by the fund's expenses, changes in securities markets, changes in the composition of the index and the timing of purchases and redemptions of fund shares.


· 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 world-wide.  Recent examples include pandemic risks related to COVID-19 and aggressive measures taken world-wide 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.  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.


· Non-diversification risk. The fund is non-diversified, which means that the fund may invest a relatively high percentage of its assets in a limited number of issuers. Therefore, the fund's performance may be more vulnerable to changes in the market value of a single issuer or group of issuers and more susceptible to risks associated with a single economic, political or regulatory occurrence than a diversified fund.

Risk Lose Money [Text] rr_RiskLoseMoney The fund's share price fluctuates, sometimes dramatically, which means you could lose money.
Risk Nondiversified Status [Text] rr_RiskNondiversifiedStatus The fund is non-diversified, which means that the fund may invest a relatively high percentage of its assets in a limited number of issuers. Therefore, the fund's performance may be more vulnerable to changes in the market value of a single issuer or group of issuers and more susceptible to risks associated with a single economic, political or regulatory occurrence than a diversified fund.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution 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 Initial shares from year to year. The table compares the average annual total returns of the fund's shares to those of a broad measure of market performance. 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.bnymellonim.com/us.


Performance information reflects the fund's expenses only and does not reflect the fees and charges imposed by participating insurance companies under their VA contracts or VLI policies. Because these fees and charges will reduce total return, policyowners should consider them when evaluating and comparing the fund's performance. Policyowners should consult the prospectus for their contract or policy for more information.
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.bnymellonim.com/us
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 (%) Initial Shares
Bar Chart Does Not Reflect Sales Loads [Text] rr_BarChartDoesNotReflectSalesLoads Performance information reflects the fund's expenses only and does not reflect the fees and charges imposed by participating insurance companies under their VA contracts or VLI policies.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock

Best Quarter
Q1, 2019: 13.59%

Worst Quarter
Q3, 2011: -13.95%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2019
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 13.59%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2011
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (13.95%)
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes reflects no deductions for fees, expenses or taxes
Average Annual Return, Caption rr_AverageAnnualReturnCaption Average Annual Total Returns (as of 12/31/19)
BNY Mellon Stock Index Fund, Inc. | S&P 500® Index reflects no deductions for fees, expenses or taxes  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 31.46%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 11.69%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 13.55%
BNY Mellon Stock Index Fund, Inc. | BNY Mellon Stock Index Fund, Inc. - Initial Shares  
Risk/Return: rr_RiskReturnAbstract  
Management fees rr_ManagementFeesOverAssets 0.25%
Distribution and/or service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses: Shareholder services fee rr_Component1OtherExpensesOverAssets none [1]
Other expenses: Miscellaneous other expenses rr_Component3OtherExpensesOverAssets 0.02%
Total other expenses rr_OtherExpensesOverAssets 0.02%
Total annual fund operating expenses rr_ExpensesOverAssets 0.27%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 28
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 87
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 152
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 343
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 28
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 87
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 152
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 343
Annual Return 2010 rr_AnnualReturn2010 14.84%
Annual Return 2011 rr_AnnualReturn2011 1.88%
Annual Return 2012 rr_AnnualReturn2012 15.74%
Annual Return 2013 rr_AnnualReturn2013 32.02%
Annual Return 2014 rr_AnnualReturn2014 13.42%
Annual Return 2015 rr_AnnualReturn2015 1.11%
Annual Return 2016 rr_AnnualReturn2016 11.71%
Annual Return 2017 rr_AnnualReturn2017 21.53%
Annual Return 2018 rr_AnnualReturn2018 (4.63%)
Annual Return 2019 rr_AnnualReturn2019 31.18%
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 31.18%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 11.42%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 13.29%
BNY Mellon Stock Index Fund, Inc. | BNY Mellon Stock Index Fund, Inc. - Service Shares  
Risk/Return: rr_RiskReturnAbstract  
Management fees rr_ManagementFeesOverAssets 0.25%
Distribution and/or service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other expenses: Shareholder services fee rr_Component1OtherExpensesOverAssets none
Other expenses: Miscellaneous other expenses rr_Component3OtherExpensesOverAssets 0.02%
Total other expenses rr_OtherExpensesOverAssets 0.02%
Total annual fund operating expenses rr_ExpensesOverAssets 0.52%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 53
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 167
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 291
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 653
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 53
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 167
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 291
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 653
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 30.84%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 11.15%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 13.01%
[1] Amount was less than .01%.