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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2017
U.S. Large Cap Equity Portfolio  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading U.S. Large Cap Equity Portfolio
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the U.S. Large Cap Equity 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 U.S. Large Cap Equity 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)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination February 28, 2018
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading PORTFOLIO TURNOVER
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The U.S. Large Cap Equity Portfolio 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 Portfolio shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the U.S. Large Cap Equity Portfolio’s performance. During the most recent fiscal year, the U.S. Large Cap Equity Portfolio’s portfolio turnover rate was 12% of the average value of its investment portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 12.00%
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 U.S. Large Cap Equity 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 U.S. Large Cap Equity Portfolio purchases a broad and diverse group of readily marketable securities of U.S. companies that the Advisor determines to be large capitalization companies within the U.S. Universe. A company’s market capitalization is the number of its shares outstanding times its price per share. The Advisor generally defines the U.S. Universe as a market capitalization weighted portfolio of U.S. operating companies listed on the New York Stock Exchange (“NYSE”), NYSE MKT LLC, Nasdaq Global Market®, Nasdaq Capital Market®, or such other securities exchanges deemed appropriate by the Advisor. As of the date of this Prospectus, for purposes of the Portfolio, the Advisor considers large cap companies to be companies whose market capitalizations are generally in the highest 90% of total market capitalization within the U.S. Universe or companies whose market capitalizations are larger than the 1,000th largest U.S. company within the U.S. Universe, whichever results in the higher market capitalization break. Under the Advisor’s market capitalization guidelines described above, based on market capitalization data as of December 31, 2016, the market capitalization of a large cap company would be $4,230 million or above. This dollar amount will change due to market conditions. As a non-fundamental policy, under normal circumstances, the U.S. Large Cap Equity Portfolio will invest at least 80% of its net assets in equity securities of large cap U.S. companies.

In addition, the Advisor may consider a company’s size, value, and/or profitability relative to other eligible companies when making investment decisions for the U.S. Large Cap Equity Portfolio. In assessing value, the Advisor may consider factors such as a company’s book value in relation to its market value, as well as price to cash flow or price to earnings ratios. In assessing profitability, the Advisor may consider factors such as that of earnings or profits from operations relative to book value or assets. The criteria the Advisor uses for assessing value or profitability are subject to change from time to time. The Advisor may also adjust the representation in the U.S. Large Cap Equity Portfolio 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.

The U.S. Large Cap Equity Portfolio may purchase or sell futures contracts and options on futures contracts for U.S. equity securities and indices, to adjust market exposure based on actual or expected cash inflows to or outflows from the Portfolio. The Portfolio does 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 U.S. Large Cap Equity Portfolio may lend its 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.

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.

Investment Strategy Risk: Securities that have high book to market ratios and/or high profitability may perform differently from the market as a whole and an investment strategy emphasizing these securities may cause the Portfolio to at times underperform equity funds that use other investment strategies.

Derivatives Risk: Derivatives are instruments, such as futures 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 U.S. Large Cap Equity Portfolio uses derivatives, the Portfolio 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 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 U.S. Large Cap Equity Portfolio may lose money and there may be a delay in recovering the loaned securities. The U.S. Large Cap Equity Portfolio could also lose money if it does 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.

Cyber Security Risk: The U.S. Large Cap Equity 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 U.S. Large Cap Equity 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 the Portfolio’s performance from year to year. The table illustrates how annualized one year and since inception returns, both before and after taxes, compare with those of a broad measure of market performance. The 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 U.S. Large Cap Equity 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 U.S. Large Cap Equity 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 the 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 Portfolio’s past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading] rr_BarChartHeading U.S. Large Cap Equity Portfolio Institutional Class Shares—Total Returns
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2014-December 2016

Highest Quarter    Lowest Quarter
5.71% (10/15–12/15)
    -7.65% (7/15–9/15)
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 U.S. Large Cap Equity 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.
U.S. Large Cap Equity Portfolio | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fee rr_ManagementFeesOverAssets 0.15%
Other Expenses rr_OtherExpensesOverAssets 0.02%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.17%
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 0.18%
1 Year rr_ExpenseExampleYear01 $ 18
3 Years rr_ExpenseExampleYear03 56
5 Years rr_ExpenseExampleYear05 97
10 Years rr_ExpenseExampleYear10 218
1 Year rr_ExpenseExampleNoRedemptionYear01 18
3 Years rr_ExpenseExampleNoRedemptionYear03 56
5 Years rr_ExpenseExampleNoRedemptionYear05 97
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 218
2014 rr_AnnualReturn2014 12.06%
2015 rr_AnnualReturn2015 (0.72%)
2016 rr_AnnualReturn2016 12.73%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Dec. 31, 2015
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 5.71%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2015
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (7.65%)
1 Year rr_AverageAnnualReturnYear01 12.73%
Since Inception rr_AverageAnnualReturnSinceInception 11.67%
Inception Date rr_AverageAnnualReturnInceptionDate Jun. 25, 2013
U.S. Large Cap Equity Portfolio | Return After Taxes on Distributions | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 12.22%
Since Inception rr_AverageAnnualReturnSinceInception 11.16%
Inception Date rr_AverageAnnualReturnInceptionDate Jun. 25, 2013
U.S. Large Cap Equity Portfolio | Return After Taxes on Distributions and Sale of Portfolio Shares | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 7.58%
Since Inception rr_AverageAnnualReturnSinceInception 9.07%
Inception Date rr_AverageAnnualReturnInceptionDate Jun. 25, 2013
U.S. Large Cap Equity Portfolio | Russell 1000® Index (reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 12.05%
Since Inception rr_AverageAnnualReturnSinceInception 12.55%
[1] Dimensional Fund Advisors LP (the “Advisor”) has agreed to waive certain fees of the U.S. Large Cap Equity Portfolio. The Amended and Restated Fee Waiver Agreement for the Portfolio will remain in effect through February 28, 2018, and may only be terminated by the Fund’s Board of Directors prior to that date. Under certain circumstances, the Advisor retains the right to seek reimbursement for any fees previously waived up to thirty-six months after such fee waiver.