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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2017
CSTG&E International Social Core Equity Portfolio  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading CSTG&E International Social Core Equity Portfolio
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the CSTG&E International Social Core Equity Portfolio (the “International Social Core 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 Social Core 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 The International Social Core 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 Portfolio’s performance. During the most recent fiscal year, the International Social Core Portfolio’s portfolio turnover rate was 5% of the average value of its investment portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 5.00%
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 Social Core 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 Social Core Portfolio purchases a broad and diverse group of securities of non-U.S. companies in developed markets with a greater emphasis on small capitalization and value companies as compared to their representation in the International Universe, while excluding companies based on the Portfolio’s social issue screens. For purposes of this Portfolio, Dimensional Fund Advisors LP (the “Advisor”) defines the International Universe as a market capitalization weighted portfolio of non-U.S. companies in developed markets that have been authorized as approved markets for investment by the Advisor’s Investment Committee. The Portfolio’s increased exposure to small capitalization and value companies may be achieved by decreasing the allocation of the International Social Core Portfolio’s assets to the largest growth companies relative to their weight in the International Universe, which would result in a greater weight allocation to small capitalization and value companies. An equity issuer is considered a growth company primarily because it has a low book value, that is not negative, in relation to its market capitalization. Securities are considered to be value stocks primarily because a company’s shares have a high book value in relation to their market value.

The International Social Core Portfolio intends to purchase securities of companies associated with developed market countries that the Advisor has designated as approved markets. As a non-fundamental policy, under normal circumstances, the International Social Core Portfolio will invest at least 80% of its net assets in equity securities. The Advisor determines company size on a country or region specific basis and based primarily on market capitalization. The percentage allocation of the assets of the International Social Core Portfolio to securities of the largest growth companies as defined above will generally be reduced from between 5% and 35% of their percentage weight in the International Universe. As of December 31, 2016, securities of the largest growth companies in the International Universe comprised approximately 15% of the International Universe and the Advisor allocated approximately 4% of the International Social Core Portfolio to securities of the largest growth companies in the International Universe. The percentage by which the Portfolio’s allocation to securities of the largest growth companies is reduced will change due to market movements, results of social issue screens and other factors. The Advisor may adjust the representation in the International Social Core 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. In assessing profitability, the Advisor may consider different ratios, such as that of earnings or profits from operations relative to book value assets.

The International Social Core Portfolio 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 Social Core Portfolio also may purchase or sell futures contracts and options on futures contracts for foreign or 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 use derivative for purposes of speculation or leveraging investment returns. The above-referenced investments are not subject to, although they may incorporate, the International Social Core Portfolio’s social criteria.

The International Social Core Portfolio may lend its portfolio securities to generate additional income.

The International Social Core Portfolio seeks to purchase securities that are consistent with the Portfolio’s social issue screens, which are monitored by an independent third party. The Portfolio seeks to exclude from its investment portfolio those companies that are identified by the Portfolio’s social issue screens. The Portfolio’s social issue screens generally are designed to identify companies that derive 15% or more of their revenue from the following industries: Casinos and Gambling; Drug retail; Brewers; Distillers and Vintners; Tobacco; Health Care Equipment and Supplies; Health Care Distributors; Health Care Facilities; Health Care Services; Managed Health Care; Health Care Technology; Biotechnology; Life Sciences Tools and Services; Pharmaceuticals; and Adult Entertainment. The Portfolio may modify this list of social issue screens, at any time, without prior shareholder approval or notice.
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 International Social Core 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.

Value Investment Risk: Value stocks may perform differently from the market as a whole and following a value-oriented investment strategy may cause the Portfolio to at times underperform equity funds that use other investment strategies.

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.

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 International Social Core Portfolio does not hedge foreign currency risk.

Social Investment Risk: The International Social Core Portfolio’s social issue screens may limit the number of investment opportunities available to the Portfolio, and as a result, at times the Portfolio may produce different returns than funds that are not subject to such special investment conditions. For example, the Portfolio may decline to purchase certain securities when it is otherwise advantageous to do so, or the Portfolio may sell certain securities for social reasons when it is otherwise disadvantageous to do so.

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 Social Core 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 International Social Core Portfolio may lose money and there may be a delay in recovering the loaned securities. The International Social Core 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 International Social Core 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 Social Core 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 International Social Core Portfolio’s performance from year to year. The table illustrates how annualized one year, five year and since inception returns, both before and after taxes, compare with those of a broad measure of market performance. The International Social Core 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 Social Core 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 Social Core 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 International Social Core 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 Social Core Portfolio’s past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading] rr_BarChartHeading CSTG&E International Social Core Equity Portfolio—Total Returns
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2008-December 2016

Highest Quarter    Lowest Quarter
30.56% (4/09–6/09)
    -21.22% (10/08–12/08)
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 Social Core 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.
CSTG&E International Social Core Equity Portfolio | CSTG&E International Social Core Equity Portfolio Shares  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fee rr_ManagementFeesOverAssets 0.42%
Other Expenses rr_OtherExpensesOverAssets 0.13%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.55%
1 Year rr_ExpenseExampleYear01 $ 56
3 Years rr_ExpenseExampleYear03 176
5 Years rr_ExpenseExampleYear05 307
10 Years rr_ExpenseExampleYear10 689
1 Year rr_ExpenseExampleNoRedemptionYear01 56
3 Years rr_ExpenseExampleNoRedemptionYear03 176
5 Years rr_ExpenseExampleNoRedemptionYear05 307
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 689
2008 rr_AnnualReturn2008 (42.88%)
2009 rr_AnnualReturn2009 36.46%
2010 rr_AnnualReturn2010 13.46%
2011 rr_AnnualReturn2011 (15.08%)
2012 rr_AnnualReturn2012 18.16%
2013 rr_AnnualReturn2013 22.93%
2014 rr_AnnualReturn2014 (7.20%)
2015 rr_AnnualReturn2015 (1.32%)
2016 rr_AnnualReturn2016 5.32%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 30.56%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (21.22%)
1 Year rr_AverageAnnualReturnYear01 5.32%
5 Years rr_AverageAnnualReturnYear05 6.97%
Since Inception rr_AverageAnnualReturnSinceInception 0.74%
Inception Date rr_AverageAnnualReturnInceptionDate Aug. 03, 2007
CSTG&E International Social Core Equity Portfolio | Return After Taxes on Distributions | CSTG&E International Social Core Equity Portfolio Shares  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 4.63%
5 Years rr_AverageAnnualReturnYear05 6.31%
Since Inception rr_AverageAnnualReturnSinceInception 0.23%
Inception Date rr_AverageAnnualReturnInceptionDate Aug. 03, 2007
CSTG&E International Social Core Equity Portfolio | Return After Taxes on Distributions and Sale of Portfolio Shares | CSTG&E International Social Core Equity Portfolio Shares  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 3.57%
5 Years rr_AverageAnnualReturnYear05 5.47%
Since Inception rr_AverageAnnualReturnSinceInception 0.63%
Inception Date rr_AverageAnnualReturnInceptionDate Aug. 03, 2007
CSTG&E International Social Core Equity Portfolio | MSCI World ex USA Index (net dividends) (reflects no deduction for fees, expenses, or taxes on sales)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 2.75%
5 Years rr_AverageAnnualReturnYear05 6.07%
Since Inception rr_AverageAnnualReturnSinceInception 0.08%