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Dreyfus Midcap Index Fund, Inc.  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Fund Summary - Dreyfus Midcap Index Fund, Inc.
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

The fund seeks to match the performance of the S&P MidCap 400® 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.
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 and may result in higher taxes when fund shares are held in a taxable account. 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 21.68% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 21.68%
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 generally is fully invested in stocks included in the S&P MidCap 400® Index and in futures whose performance is tied to the index. The fund generally invests in all 400 stocks in the S&P MidCap 400® Index in proportion to their weighting in the index.


The S&P MidCap 400® Index is an unmanaged index of 400 common stocks of medium-size companies. 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 MidCap 400® Index generally have market capitalizations ranging between approximately $1.4 billion and $5.9 billion, to the extent consistent with market conditions.


"Standard & Poor's®," "S&P®" and "S&P MidCap 400® Index" are trademarks of Standard & Poor's Financial Services LLC 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.


· Small and midsize company risk. Small and midsize companies carry additional risks because the operating histories of these companies tend to be more limited, their earnings and revenues less predictable (and some companies may be experiencing significant losses), and their share prices more volatile than those of larger, more established companies. The shares of smaller companies tend to trade less frequently than those of larger, more established companies, which can adversely affect the pricing of these securities and the fund's ability to sell these securities.


· 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 Investor 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 (before and after taxes) 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. Prior to August 31, 2016, the fund's Investor shares were undesignated.

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 (before and after taxes) 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 (%) Investor Shares
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock

Best Quarter
Q3, 2009: 19.81%


Worst Quarter
Q4, 2008: -25.61%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Sep. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 19.81%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (25.61%)
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes reflects no deductions for fees, expenses or taxes
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate After-tax returns are calculated using the historical highest individual federal marginal income tax rates, and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred Actual after-tax returns depend on the investor's tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through U.S. tax-deferred arrangements such as 401(k) plans or individual retirement accounts.
Performance Table One Class of after Tax Shown [Text] rr_PerformanceTableOneClassOfAfterTaxShown After-tax performance is shown only for Investor shares. After-tax performance of the fund's other share class will vary.
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock

After-tax performance is shown only for Investor shares. After-tax performance of the fund's other share class will vary. After-tax returns are calculated using the historical highest individual federal marginal income tax rates, and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor's tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through U.S. tax-deferred arrangements such as 401(k) plans or individual retirement accounts.


For the fund's Class I shares, periods prior to the inception date reflect the performance of the fund's Investor shares. Such performance figures have not been adjusted to reflect applicable class fees and expenses. Each share class is invested in the same portfolio of securities, and the annual returns would have differed only to the extent that the classes do not have the same expenses.
Caption rr_AverageAnnualReturnCaption Average Annual Total Returns (as of 12/31/16)
Dreyfus Midcap Index Fund, Inc. | S&P MidCap 400® Index reflects no deductions for fees, expenses or taxes  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 20.74%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 15.33%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 9.16%
Dreyfus Midcap Index Fund, Inc. | Investor Shares  
Risk/Return: rr_RiskReturnAbstract  
Management fees rr_ManagementFeesOverAssets 0.25%
Other expenses (including shareholder services fees) rr_OtherExpensesOverAssets 0.26%
Total annual fund operating expenses rr_ExpensesOverAssets 0.51%
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.01%) [1]
Total annual fund operating expenses (after fee waiver and/or expense reimbursement) rr_NetExpensesOverAssets 0.50%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 51
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 160
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 280
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 628
Annual Return 2007 rr_AnnualReturn2007 7.56%
Annual Return 2008 rr_AnnualReturn2008 (36.45%)
Annual Return 2009 rr_AnnualReturn2009 37.04%
Annual Return 2010 rr_AnnualReturn2010 26.03%
Annual Return 2011 rr_AnnualReturn2011 (2.20%)
Annual Return 2012 rr_AnnualReturn2012 17.21%
Annual Return 2013 rr_AnnualReturn2013 32.86%
Annual Return 2014 rr_AnnualReturn2014 9.37%
Annual Return 2015 rr_AnnualReturn2015 (2.63%)
Annual Return 2016 rr_AnnualReturn2016 20.23%
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 20.23%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 14.80%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 8.70%
Dreyfus Midcap Index Fund, Inc. | Investor Shares | After Taxes on Distributions  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 17.43%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 12.85%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 7.32%
Dreyfus Midcap Index Fund, Inc. | Investor Shares | After Taxes on Distributions and Sale of Fund Shares  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 13.74%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 11.76%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 6.93%
Dreyfus Midcap Index Fund, Inc. | Class I  
Risk/Return: rr_RiskReturnAbstract  
Management fees rr_ManagementFeesOverAssets 0.25%
Other expenses (including shareholder services fees) rr_OtherExpensesOverAssets 0.01%
Total annual fund operating expenses rr_ExpensesOverAssets 0.26%
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.01%) [1]
Total annual fund operating expenses (after fee waiver and/or expense reimbursement) rr_NetExpensesOverAssets 0.25%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 26
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 80
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 141
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 318
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 20.29%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 14.81%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 8.70%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Aug. 31, 2016
[1] The fund's investment adviser, The Dreyfus Corporation, has agreed in its management agreement with the fund to: (1) pay all of the fund's expenses, except management fees, shareholder services plan fees and certain other expenses, including the fees and expenses of the non-interested board members and the fees and expenses of counsel to the fund and to the non-interested board members, and (2) reduce its fees pursuant to the management agreement in an amount equal to the fund's allocable portion of the fees and expenses of the non-interested board members and the fees and expenses of counsel to the fund and to the non-interested board members (in the amount of .01% for the past fiscal year). These provisions in the management agreement may not be amended without the approval of the fund's shareholders.