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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2017
INTERNATIONAL SMALL COMPANY PORTFOLIO  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading International Small Company Portfolio
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the International Small Company Portfolio is to achieve long-term capital appreciation.
Expense [Heading] rr_ExpenseHeading Fees and Expenses of the Portfolio
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock This table describes the fees and expenses you may pay if you buy and hold shares of the International Small Company Portfolio.
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)
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading PORTFOLIO TURNOVER
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock A mutual fund generally 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 mutual fund shares are held in a taxable account. The International Small Company Portfolio does not pay transaction costs when buying and selling shares of other mutual funds (the “Underlying Funds”); however, the Underlying Funds pay transaction costs when buying and selling securities for their portfolio. The transaction costs incurred by the Underlying Funds, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the International Small Company Portfolio’s performance. During the most recent fiscal year, the International Small Company Portfolio’s portfolio turnover rate was 10% based on the weighted average portfolio turnover ratios of each of the Portfolio’s underlying investments.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 10.00%
Expense Example [Heading] rr_ExpenseExampleHeading EXAMPLE
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This Example is meant to help you compare the cost of investing in the International Small Company Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated. The Example also assumes that your investment has a 5% return each year and that the Portfolio’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 Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock The International Small Company Portfolio is a “fund of funds,” which means the Portfolio generally allocates its assets among other funds managed by Dimensional Fund Advisors LP (the “Advisor”) (the “Underlying Funds”), although it has the ability to invest directly in securities and derivatives. The International Small Company Portfolio seeks to achieve its investment objective of providing investors with access to securities portfolios consisting of a broad range of equity securities of primarily small Canadian, Japanese, United Kingdom, Continental European and Asia Pacific companies. The International Small Company Portfolio also may have some exposure to small cap equity securities associated with other countries or regions. The International Small Company Portfolio pursues its investment objective by investing substantially all of its assets in the following Underlying Funds: The Canadian Small Company Series, The Japanese Small Company Series, The Asia Pacific Small Company Series, The United Kingdom Small Company Series and The Continental Small Company Series of The DFA Investment Trust Company. Periodically, the Advisor will review the allocations for the International Small Company Portfolio in each Underlying Fund and may adjust allocations to the Underlying Funds or may add or remove Underlying Funds in the Portfolio without notice to shareholders. Each Underlying Fund invests in small companies using a market capitalization weighted approach in each country or region designated by the Advisor as an approved market for investment. A company’s market capitalization is the number of its shares outstanding times its price per share. In general, the higher the relative market capitalization of a small company within an eligible country, the greater its representation in the Underlying Fund. The Advisor may adjust the representation in the Underlying Funds of an eligible company, or exclude a company, after considering such factors as free float, momentum, trading strategies, liquidity, profitability, and other factors that the Advisor determines to be appropriate, given market conditions. In assessing profitability, the Advisor may consider different ratios, such as that of earnings or profits from operations relative to book value or assets.

As a non-fundamental policy, under normal circumstances, the International Small Company Portfolio, through its investments in the Underlying Funds, will invest at least 80% of its net assets in securities of small companies. The International Small Company Portfolio and each Underlying Fund may invest in affiliated and unaffiliated registered and unregistered money market funds to manage its cash pending investment in other securities or to maintain liquidity for the payment of redemptions or other purposes. Investments in money market funds may involve a duplication of certain fees and expenses.

Each Underlying Fund may gain exposure to companies associated with approved markets by purchasing equity securities in the form of depositary receipts, which may be listed or traded outside the issuer’s domicile country. The International Small Company Portfolio and each Underlying Fund may purchase or sell futures contracts and options on futures contracts for equity securities and indices of its approved markets or other equity market securities or indices, including those of the United States, to adjust market exposure based on actual or expected cash inflows to or outflows from the Portfolio or Underlying Fund. The International Small Company Portfolio and Underlying Funds do not intend to sell futures contracts to establish short positions in individual securities or to use derivatives for purposes of speculation or leveraging investment returns.

The International Small Company Portfolio and the Underlying Funds may lend their portfolio securities to generate additional income.
Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock Because the value of your investment in the Portfolio will fluctuate, there is the risk that you will lose money. The following is a description of principal risks of investing in the Portfolio.

Fund of Funds Risk: The investment performance of the International Small Company Portfolio is affected by the investment performance of the Underlying Funds in which the International Small Company Portfolio invests. The ability of the International Small Company Portfolio to achieve its investment objective depends on the ability of the Underlying Funds to meet their investment objectives and on the Advisor’s decisions regarding the allocation of the Portfolio’s assets among the Underlying Funds. There can be no assurance that the investment objective of the International Small Company Portfolio or any Underlying Fund will be achieved. When the Portfolio invests in Underlying Funds, investors are exposed to a proportionate share of the expenses of those Underlying Funds in addition to the expenses of the Portfolio. Through its investments in the Underlying Funds, the International Small Company Portfolio is subject to the risks of the Underlying Funds’ investments. The risks of the International Small Company Portfolio’s and Underlying Funds’ investments are described below.

Equity Market Risk: Even a long-term investment approach cannot guarantee a profit. Economic, market, political, and issuer-specific conditions and events will cause the value of equity securities, and the Portfolio that owns them, to rise or fall. Stock markets tend to move in cycles, with periods of rising prices and periods of falling prices.

Foreign Securities and Currencies Risk: Foreign securities prices may decline or fluctuate because of: (a) economic or political actions of foreign governments, and/or (b) less regulated or liquid securities markets. Investors holding these securities may also be exposed to foreign currency risk (the possibility that foreign currency will fluctuate in value against the U.S. dollar or that a foreign government will convert, or be forced to convert, its currency to another currency, changing its value against the U.S. dollar). The Underlying Funds do not hedge foreign currency risk.

Small Company Risk: Securities of small companies are often less liquid than those of large companies and this could make it difficult to sell a small company security at a desired time or price. As a result, small company stocks may fluctuate relatively more in price. In general, smaller capitalization companies are also more vulnerable than larger companies to adverse business or economic developments and they may have more limited resources.

Derivatives Risk: Derivatives are instruments, such as futures and foreign exchange forward contracts, whose value is derived from that of other assets, rates or indices. The use of derivatives for non-hedging purposes may be considered to carry more risk than other types of investments. When the International Small Company Portfolio or an Underlying Fund uses derivatives, the Portfolio or Underlying Fund will be directly exposed to the risks of those derivatives. Derivative instruments are subject to a number of risks including counterparty, liquidity, interest rate, market, credit and management risks, and the risk of improper valuation. Changes in the value of a derivative may not correlate perfectly with the underlying asset, rate or index, and the Portfolio or Underlying Fund could lose more than the principal amount invested.

Securities Lending Risk: Securities lending involves the risk that the borrower may fail to return the securities in a timely manner or at all. As a result, the Underlying Funds may lose money and there may be a delay in recovering the loaned securities. The Underlying Funds could also lose money if they do not recover the securities and/or the value of the collateral falls, including the value of investments made with cash collateral. Securities lending also may have certain adverse tax consequences. To the extent that the Portfolio holds securities directly and lends those securities, it will be also subject to the foregoing risks with respect to its loaned securities.

Cyber Security Risk: The International Small Company Portfolio’s and its service providers’ use of internet, technology and information systems may expose the Portfolio to potential risks linked to cyber security breaches of those technological or information systems. Cyber security breaches, amongst other things, could allow an unauthorized party to gain access to proprietary information, customer data, or fund assets, or cause the Portfolio and/or its service providers to suffer data corruption or lose operational functionality.
Risk Lose Money [Text] rr_RiskLoseMoney Because the value of your investment in the Portfolio will fluctuate, there is the risk that you will lose money.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The bar chart and table immediately following illustrate the variability of the International Small Company Portfolio’s returns and are meant to provide some indication of the risks of investing in the Portfolio. The bar chart shows the changes in International Small Company Portfolio’s performance from year to year. The table illustrates how annualized one year, five year and ten year returns, both before and after taxes, compare with those of a broad measure of market performance. The International Small Company Portfolio’s past performance (before and after taxes) is not an indication of future results. Updated performance information for the Portfolio can be obtained by visiting http://us.dimensional.com.

The after-tax returns presented in the table for the International Small Company Portfolio 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 an investor’s tax situation and may differ from those shown in the table. In addition, the after-tax returns shown are not relevant to investors who hold shares of the Portfolio through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The bar chart and table immediately following illustrate the variability of the International Small Company Portfolio’s returns and are meant to provide some indication of the risks of investing in the Portfolio. The bar chart shows the changes in International Small Company Portfolio’s performance from year to year.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress http://us.dimensional.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The International Small Company Portfolio’s past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading] rr_BarChartHeading International Small Company Portfolio Institutional Class Shares—Total Returns
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2007-December 2016

Highest Quarter    Lowest Quarter
31.49% (4/09–6/09)
    -22.43% (7/08–9/08)
Performance Table Heading rr_PerformanceTableHeading Annualized Returns (%)
Periods ending December 31, 2016
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate The after-tax returns presented in the table for the International Small Company Portfolio 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 In addition, the after-tax returns shown are not relevant to investors who hold shares of the Portfolio through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts.
INTERNATIONAL SMALL COMPANY PORTFOLIO | INSTITUTIONAL CLASS  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fee rr_ManagementFeesOverAssets 0.40%
Other Expenses rr_OtherExpensesOverAssets 0.01%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.12%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.53%
1 Year rr_ExpenseExampleYear01 $ 54
3 Years rr_ExpenseExampleYear03 170
5 Years rr_ExpenseExampleYear05 296
10 Years rr_ExpenseExampleYear10 665
1 Year rr_ExpenseExampleNoRedemptionYear01 54
3 Years rr_ExpenseExampleNoRedemptionYear03 170
5 Years rr_ExpenseExampleNoRedemptionYear05 296
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 665
2007 rr_AnnualReturn2007 5.66%
2008 rr_AnnualReturn2008 (43.87%)
2009 rr_AnnualReturn2009 41.96%
2010 rr_AnnualReturn2010 23.91%
2011 rr_AnnualReturn2011 (15.35%)
2012 rr_AnnualReturn2012 18.85%
2013 rr_AnnualReturn2013 27.44%
2014 rr_AnnualReturn2014 (6.30%)
2015 rr_AnnualReturn2015 5.91%
2016 rr_AnnualReturn2016 5.80%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 31.49%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (22.43%)
1 Year rr_AverageAnnualReturnYear01 5.80%
5 Years rr_AverageAnnualReturnYear05 9.72%
10 Years rr_AverageAnnualReturnYear10 3.46%
INTERNATIONAL SMALL COMPANY PORTFOLIO | Return After Taxes on Distributions | INSTITUTIONAL CLASS  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 4.41%
5 Years rr_AverageAnnualReturnYear05 8.41%
10 Years rr_AverageAnnualReturnYear10 2.46%
INTERNATIONAL SMALL COMPANY PORTFOLIO | Return After Taxes on Distributions and Sale of Portfolio Shares | INSTITUTIONAL CLASS  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 4.23%
5 Years rr_AverageAnnualReturnYear05 7.52%
10 Years rr_AverageAnnualReturnYear10 2.63%
INTERNATIONAL SMALL COMPANY PORTFOLIO | MSCI World ex USA Small Cap Index (net dividends) (reflects no deduction for fees, expenses, or taxes on sales)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 4.32%
5 Years rr_AverageAnnualReturnYear05 8.96%
10 Years rr_AverageAnnualReturnYear10 2.69%