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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName MUTUAL OF AMERICA INVESTMENT CORP
Prospectus Date rr_ProspectusDate May 01, 2020
Small Cap Equity Index Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Small Cap Equity Index Fund May 1, 2020
Objective [Heading] rr_ObjectiveHeading Investment Objective.
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The Fund seeks investment results that correspond to the investment performance of the S&P SmallCap 600® Index.
Expense [Heading] rr_ExpenseHeading Fees and Expenses of the Fund.
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock The table below describes the fees and expenses you may pay if you buy and hold Fund shares. For investments through an insurance company separate account, note that the expenses shown do not include Separate Account expenses or variable contract expenses which would increase costs if included.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption Shareholder Fees (fees paid directly from your investment)
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)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination April 30, 2021
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 rate may result in 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 Fund’s most recently completed fisal year, the Fund’s portfolio turnover rate was 97.06% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 97.06%
Expense Example [Heading] rr_ExpenseExampleHeading Example.
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This 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: (a) that you invest $10,000 in the Fund for the time periods indicated and then redeem or hold all of your shares at the end of those periods, (b) a 5% return each year and (c) operating expenses remain the same. The Example reflects the expense reimbursement for the first year. For investments through an insurance company separate account, note that the expenses shown do not include Separate Account expenses or variable contract expenses which would increase costs if included. Although your actual costs may be higher or lower, your cost based on these assumptions would be:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies.
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock The Fund invests primarily in the 600 common stocks included in the S&P SmallCap 600® Index to replicate, to the extent practicable, the weightings of such stocks in the Index. The components of the Index are market capitalization weighted, adjusted for free float, which is the value of shares readily available in the market as held by public investors. Under normal circumstances, at least 80% of the Fund’s total assets are invested in securities included in the S&P SmallCap 600® Index, which as of December 31, 2019, included companies with market capitalizations from $97.98 million up to $6.8 billion. The Fund is rebalanced at approximately the same time that the S&P SmallCap 600® Index to which the Fund is benchmarked is rebalanced and reconstituted, which currently occurs on the third Friday of the last month of each quarter. The fund will concentrate its investments in an industry or group of industries to the extent that the index being tracked is also so concentrated.
Risk [Heading] rr_RiskHeading Principal Investment Risks.
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock As with any mutual fund, loss of money is a risk of investing in the Fund. Additionally, an investment in the Fund is subject to the following risks which are described in more detail in the Prospectus.
  • General risk: The Fund may not achieve its investment objective. An investment in the Fund could decline in value, and you could lose money by investing in the Fund.
  • Market risk: The risk that prices of securities will go down because of the interplay of market forces may affect a single issuer, industry or sector of the economy or may affect the market as a whole. Events such as war, acts of terrorism, social unrest, natural disasters, public health emergencies and other unforeseen events could also significantly impact issuers, economies and markets generally.
  • Company risk: The price of the stock of a particular company can vary based on a variety of factors, such as the company’s financial performance, changes in management and product trends, and the potential for takeover and acquisition. The prices of equity securities of smaller companies may fluctuate more than for more established companies. The equity securities of smaller companies may not be traded as often as for larger companies, therefore it may be difficult to trade securities at a desirable price. Investments in companies with small market capitalizations generally offer greater opportunities for appreciation, but are associated with more risks than for established companies.
  • Concentration risk: The chance that the stocks of a particular industry or group of industries will decline because of adverse developments affecting that industry. Because the Fund concentrates its assets in certain industries to the same extent that the S&P SmallCap 600® Index does, the Fund bears concentration risk.
  • Small-Cap risk: Small-cap stocks generally are subject to greater, less predictable price changes than the securities of companies with larger market capitalizations.
  • Index Tracking Error risk: As an index fund, the Fund seeks to track the performance of the index, although it may not be successful in doing so. The divergence between the performance of the Fund and the index, positive or negative, is called tracking error. Tracking error can be caused by many factors such as operating and transaction costs, as well as weighting of each security in the index, and it may be significant.
  • Passive Investment risk: Because the Fund is passively managed and seeks to match the performance of its benchmark index, holdings are generally not reallocated based on changes in market conditions or outlook for a specific security, industry, or market sector. As a result, the Fund’s performance may lag the performance of actively managed funds.
  • Stock risk: The value of your investment will go up or down, depending on movements in the stock markets. The investment results may be better or worse than the results for the stock markets taken as a whole, or than the results of other funds that invest in the same types of securities.
Risk Lose Money [Text] rr_RiskLoseMoney An investment in the Fund could decline in value, and you could lose money by investing in the Fund.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance/Annual Return.
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The bar chart and table below show the annual return and average annual returns of the Fund.

Below the bar chart are the Fund’s highest and lowest total returns for any calendar quarter during the period covered by the chart, showing the volatility of the Fund’s total returns. The numbers in parentheses are negative, representing a loss of principal.

The information indicates some of the risks of investing in the Fund by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for one year and the life of the Fund compare to those of a broad-based, unmanaged index for those periods. A fund’s past performance does not necessarily indicate how it will perform in the future.

For investments through an insurance company separate account, note that the total returns and average annual total returns shown do not include charges against the assets of the Separate Accounts that purchase Fund shares. If these charges were reflected, returns would be less than those shown. Updated performance information is available at no cost online at http://www.mutualofamerica.com or by calling 1-800-468-3785.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The bar chart and table below show the annual return and average annual returns of the Fund. The information indicates some of the risks of investing in the Fund by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for one year and the life of the Fund compare to those of a broad-based, unmanaged index for those periods.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone 1-800-468-3785
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress http://www.mutualofamerica.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture A fund’s past performance does not necessarily indicate how it will perform in the future.
Bar Chart Does Not Reflect Sales Loads [Text] rr_BarChartDoesNotReflectSalesLoads For investments through an insurance company separate account, note that the total returns and average annual total returns shown do not include charges against the assets of the Separate Accounts that purchase Fund shares. If these charges were reflected, returns would be less than those shown.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock Best and Worst Performing Quarters

     Quarter/Year    Total Return  
Best   
First quarter 2019
      11.53 % 
Worst   
Third quarter 2019
      (0.28 )% 
Performance Table Heading rr_PerformanceTableHeading Average Annual Total Returns (for periods ended December 31, 2019)
Small Cap Equity Index Fund | Small Cap Equity Index Fund Class  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther
Management Fees rr_ManagementFeesOverAssets 0.08%
Other Expenses rr_OtherExpensesOverAssets 0.35%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.43%
Expense Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.28%) [1]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 0.15%
1 Year rr_ExpenseExampleYear01 $ 15
3 Years rr_ExpenseExampleYear03 110
5 Years rr_ExpenseExampleYear05 215
10 Years rr_ExpenseExampleYear10 526
1 Year rr_ExpenseExampleNoRedemptionYear01 15
3 Years rr_ExpenseExampleNoRedemptionYear03 110
5 Years rr_ExpenseExampleNoRedemptionYear05 215
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 526
2019 rr_AnnualReturn2019 22.40%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2019
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 11.53%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2019
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (0.28%)
Past One Year rr_AverageAnnualReturnYear01 22.40%
For Life of Fund rr_AverageAnnualReturnSinceInception 1.40%
Small Cap Equity Index Fund | S&P SmallCap 600® Index (Index reflects no deduction for fees and expenses)  
Risk/Return: rr_RiskReturnAbstract  
Past One Year rr_AverageAnnualReturnYear01 22.78%
For Life of Fund rr_AverageAnnualReturnSinceInception 1.83%
[1] The Adviser has contractually agreed beginning as of the inception of the Fund to reimburse the Fund’s direct operating expenses to the extent such operating expenses exceed 0.07% (excluding any extraordinary expenses that may arise and charges incurred in trading portfolio securities). This contractual obligation may not be terminated before April 30, 2021, and will continue for each succeeding 12 month period thereafter, unless either the Investment Company (at the direction of its Board of Directors) gives not less than 30 days’ notice of termination to the Adviser or the Adviser gives written notice of termination to the Investment Company within a 45 calendar day period prior to the next May 1.