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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2018
U.S. CORE EQUITY 2 PORTFOLIO  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading U.S. Core Equity 2 Portfolio
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the U.S. Core Equity 2 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. Core Equity 2 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 U.S. Core Equity 2 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. Core Equity 2 Portfolio’s performance. During the most recent fiscal year, the U.S. Core Equity 2 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 U.S. Core Equity 2 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. Core Equity 2 Portfolio purchases a broad and diverse group of securities of U.S. companies with a greater emphasis on small capitalization, value and high profitability companies as compared to their representation in the U.S. Universe. Dimensional Fund Advisors LP (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 American LLC, Nasdaq Global Market®, Nasdaq Capital Market®, or such other securities exchanges deemed appropriate by the Advisor. The Portfolio’s increased exposure to small capitalization, value and high profitability companies may be achieved by decreasing the allocation of the Portfolio’s assets to the largest U.S. growth or low profitability companies relative to their weight in the U.S. Universe, which would result in a greater weight allocation to small capitalization, value and/or high profitability companies. An equity issuer is considered a growth company primarily because it has a high price in relation to its book value. Securities are considered value stocks primarily because a company’s shares have a low price in relation to their book value. In assessing growth and value, the Advisor may consider additional factors such as price to cash flow or price to earnings ratios. In assessing profitability, the Advisor may consider different ratios, such as that of earnings or profits from operations relative to book value or assets. The criteria the Advisor uses for assessing growth, value, or profitability are subject to change from time to time.

As a non-fundamental policy, under normal circumstances, U.S. Core Equity 2 Portfolio will invest at least 80% of its net assets in equity securities of U.S. companies. The percentage allocation of the assets of the U.S. Core Equity 2 Portfolio to securities of the largest U.S. growth companies as defined above will generally be reduced from between 5% and 35% of their percentage weight in the U.S. Universe. For example, as of December 31, 2017, securities of the largest U.S. growth companies comprised 33% of the U.S. Universe and the Advisor allocated approximately 16% of the U.S. Core Equity 2 Portfolio to securities of the largest U.S. growth companies. The percentage by which the U.S. Core Equity 2 Portfolio’s allocation to securities of the largest U.S. growth companies is reduced will change due to market movements. The Advisor may also adjust the representation in the U.S. Core Equity 2 Portfolio of an eligible company, or exclude a company, after considering such factors as free float, momentum, trading strategies, liquidity, size, value, profitability, and other factors that the Advisor determines to be appropriate, given market conditions.

The U.S. Core Equity 2 Portfolio also 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. Core Equity 2 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. An investment in the Portfolio is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. 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.

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.

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.

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. Core Equity 2 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. Core Equity 2 Portfolio may lose money and there may be a delay in recovering the loaned securities. The U.S. Core Equity 2 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. Core Equity 2 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.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution An investment in the Portfolio is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
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. Core Equity 2 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 U.S. Core Equity 2 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 U.S. Core Equity 2 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. Core Equity 2 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. Core Equity 2 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 U.S. Core Equity 2 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 U.S. Core Equity 2 Portfolio’s past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading] rr_BarChartHeading U.S. Core Equity 2 Portfolio Institutional Class Shares—Total Returns
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2008-December 2017

Highest Quarter    Lowest Quarter
19.26% (4/09–6/09)
    -24.17% (10/08–12/08)
Performance Table Heading rr_PerformanceTableHeading Annualized Returns (%)
Periods ending December 31, 2017
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate The after-tax returns presented in the table for the U.S. Core Equity 2 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. CORE EQUITY 2 PORTFOLIO | INSTITUTIONAL CLASS  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fee rr_ManagementFeesOverAssets 0.20%
Other Expenses rr_OtherExpensesOverAssets 0.02%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.22%
1 Year rr_ExpenseExampleYear01 $ 23
3 Years rr_ExpenseExampleYear03 71
5 Years rr_ExpenseExampleYear05 124
10 Years rr_ExpenseExampleYear10 280
1 Year rr_ExpenseExampleNoRedemptionYear01 23
3 Years rr_ExpenseExampleNoRedemptionYear03 71
5 Years rr_ExpenseExampleNoRedemptionYear05 124
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 280
2008 rr_AnnualReturn2008 (36.86%)
2009 rr_AnnualReturn2009 29.15%
2010 rr_AnnualReturn2010 21.81%
2011 rr_AnnualReturn2011 (2.09%)
2012 rr_AnnualReturn2012 18.08%
2013 rr_AnnualReturn2013 37.76%
2014 rr_AnnualReturn2014 9.32%
2015 rr_AnnualReturn2015 (3.07%)
2016 rr_AnnualReturn2016 16.58%
2017 rr_AnnualReturn2017 18.94%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 19.26%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (24.17%)
1 Year rr_AverageAnnualReturnYear01 18.94%
5 Years rr_AverageAnnualReturnYear05 15.14%
10 Years rr_AverageAnnualReturnYear10 8.80%
U.S. CORE EQUITY 2 PORTFOLIO | Return After Taxes on Distributions | INSTITUTIONAL CLASS  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 18.13%
5 Years rr_AverageAnnualReturnYear05 14.39%
10 Years rr_AverageAnnualReturnYear10 8.27%
U.S. CORE EQUITY 2 PORTFOLIO | Return After Taxes on Distributions and Sale of Portfolio Shares | INSTITUTIONAL CLASS  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 11.21%
5 Years rr_AverageAnnualReturnYear05 12.05%
10 Years rr_AverageAnnualReturnYear10 7.07%
U.S. CORE EQUITY 2 PORTFOLIO | Russell 3000® Index (reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 21.13%
5 Years rr_AverageAnnualReturnYear05 15.58%
10 Years rr_AverageAnnualReturnYear10 8.60%