Label

Element

Value

Risk/Return:

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Registrant Name

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MUTUAL OF AMERICA INVESTMENT CORP

Prospectus Date

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May 01, 2015

Mid-Cap Equity Index Fund

Risk/Return:

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Risk/Return [Heading]

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Mid-Cap Equity Index Fund May 1, 2015

Objective [Heading]

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Investment Objective.

Objective, Primary [Text Block]

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The Fund seeks investment results that correspond to the investment performance of the S&P MidCap 400^® Index.

Expense [Heading]

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Fees and Expenses of the Fund.

Expense Narrative [Text Block]

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The table below describes the fees and expenses you may pay if you buy and hold Fund shares. The expenses shown do not include Separate Account expenses which would increase costs if included.

Shareholder Fees Caption [Text]

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Shareholder Fees (fees paid directly from your investment)

Operating Expenses Caption [Text]

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Annual Fund Operating Expenses
(expenses that you pay each year as a percentage of the value of your investment)

Portfolio Turnover [Heading]

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Portfolio Turnover.

Portfolio Turnover [Text Block]

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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 fiscal year, the Fund’s portfolio turnover rate was 16.29% of the average value of its portfolio.

Portfolio Turnover, Rate

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16.29%

Expense Example [Heading]

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Example.

Expense Example Narrative [Text Block]

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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 all of your shares at the end of those periods, (b) a 5% return each year and (c) operating expenses remain the same. The expenses shown do not include Separate Account 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]

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Principal Investment Strategies.

Strategy Narrative [Text Block]

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The Fund invests primarily in the 400 common stocks included in the S&P MidCap 400^® Index to replicate, to the extent practicable, the weightings of such stocks in the Index. The Fund also purchases futures contracts on the S&P MidCap 400^® Index. Under normal circumstances, at least 80% of the Fund’s total assets are invested in securities included in the S&P 400^® Index.

Risk [Heading]

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Principal Investment Risks.

Risk Narrative [Text Block]

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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.

• 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.

• 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.

• Index risk: The Fund’s investment performance may not precisely duplicate the performance of the index. The Fund may rebalance the portfolio to account for changes in the composition of the index or in the valuations of the stocks within the index.

• Mid-Cap risk: Mid-cap stocks experience more market risk and sharper price fluctuations than for large-cap stocks due to the fact that the earnings of mid-size companies tend to be less predictable and the stocks are traded less frequently. At times it may be difficult for a Fund to sell mid-cap stocks at a price equal to their value.

• Liquidity risk: The Fund’s investments in illiquid securities, investments that are difficult to purchase or sell, may reduce returns if the Fund is unable to sell at an advantageous time or price. Foreign securities or securities with substantial market and/or credit risk and securities that trade over-the-counter tend to have the greatest exposure to liquidity risk.

• Options and Futures risk: Securities held in the Fund may not be exactly the same as securities in the underlying options or futures contracts and as a result the price of the securities being hedged may not move in the same amount or direction as the underlying index, securities or debt obligation. When the Fund purchases an option, it may lose the entire premium plus costs. When an option is exercised the Fund may be required to sell the security at a price below its market value.

• 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. It may be more difficult for the Fund to sell a mid-capitalization stock or any stock that trades “over-the-counter,” than a larger capitalization stock or stocks that trade on a national or regional stock exchange.

Risk Lose Money [Text]

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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]

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Performance/Annual Return.

Performance Narrative [Text Block]

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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 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, five, and ten years 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.

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]

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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, five, and ten years compare to those of a broad-based, unmanaged index for those periods.

Performance Availability Phone [Text]

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1-800-468-3785

Performance Availability Website Address [Text]

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http://www.mutualofamerica.com

Performance Past Does Not Indicate Future [Text]

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A fund’s past performance does not necessarily indicate how it will perform in the future.

Bar Chart Does Not Reflect Sales Loads [Text]

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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]

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Best and Worst Performing Quarters

	Quarter/Year	Total Return
Best	Third quarter 2009		19.87	%
Worst	Fourth quarter 2008		(25.57	)%

Performance Table Heading

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Average Annual Total Returns (for periods ended December 31, 2014)

Mid-Cap Equity Index Fund | Mid-Cap Equity Index Fund Class

Risk/Return:

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Shareholder Fees (fees paid directly from your investment)

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Management Fees

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0.08%

Other Expenses

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0.09%

Total Annual Fund Operating Expenses

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0.17%

1 Year

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17

3 Years

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55

5 Years

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96

10 Years

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219

2005

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12.50%

2006

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10.11%

2007

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7.74%

2008

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(36.31%)

2009

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36.69%

2010

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26.28%

2011

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(1.99%)

2012

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17.80%

2013

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33.21%

2014

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9.63%

Highest Quarterly Return, Label

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Best

Highest Quarterly Return, Date

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Sep. 30, 2009

Highest Quarterly Return

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19.87%

Lowest Quarterly Return, Label

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Worst

Lowest Quarterly Return, Date

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Dec. 31, 2008

Lowest Quarterly Return

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(25.57%)

Past One Year

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9.63%

Past Five Years

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16.31%

Past Ten Years

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9.47%

Mid-Cap Equity Index Fund | S&P MidCap 400® Index (Index reflects no deduction for fees and expenses)

Risk/Return:

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Past One Year

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9.77%

Past Five Years

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16.54%

Past Ten Years

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9.71%