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Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2020
U.S. SMALL CAP VALUE PORTFOLIO  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading U.S. Small Cap Value Portfolio
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the U.S. Small Cap Value 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. Small Cap Value 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. Small Cap Value 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. Small Cap Value Portfolio’s performance. During the most recent fiscal year, the U.S. Small Cap Value Portfolio’s portfolio turnover rate was 19% of the average value of its investment portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 19.00%
Expenses Restated to Reflect Current [Text] rr_ExpensesRestatedToReflectCurrent The “Management Fee” and “Total Annual Fund Operating Expenses” have been adjusted to reflect the decrease in the management fee payable by the Portfolio from 0.50% to 0.48% effective as of February 28, 2020.
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. Small Cap Value 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 U.S. Small Cap Value Portfolio, using a market capitalization weighted approach, purchases a broad and diverse group of the readily marketable securities of U.S. small cap companies that Dimensional Fund Advisors LP (the “Advisor”) determines to be value stocks. 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 the U.S. small cap company, the greater its representation in the Portfolio. The Advisor may adjust the representation in the U.S. Small Cap Value Portfolio of an eligible company, or exclude a company, after considering such factors as free float, momentum, trading strategies, liquidity, value, profitability, investment characteristics, and other factors that the Advisor determines to be appropriate. Securities are considered value stocks primarily because a company’s shares have a low price in relation to their book value. In assessing 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. In assessing a company’s investment characteristics, the Advisor may consider ratios such as recent changes in assets or book value scaled by assets or book value. The criteria the Advisor uses for assessing value, profitability, or investment characteristics are subject to change from time to time. The Advisor may also adjust the representation in the Portfolio of an eligible company, or exclude a company, that the Advisor believes to be negatively impacted by environmental, social or governance factors (including accounting practices and shareholder rights) to a greater degree relative to other issuers.

As a non-fundamental policy, under normal circumstances, the U.S. Small Cap Value Portfolio will invest at least 80% of its net assets in securities of small cap U.S. companies. As of the date of this Prospectus, for purposes of the U.S. Small Cap Value Portfolio, the Advisor considers small cap companies to be companies whose market capitalizations are generally in the lowest 10% of total market capitalization or companies whose market capitalizations are smaller than the 1,000th largest U.S. company, whichever results in the higher market capitalization break. Total market capitalization is based on the market capitalization of eligible U.S. operating companies listed on a securities exchange in the United States that is deemed appropriate by the Advisor. Under the Advisor’s market capitalization guidelines described above, based on market capitalization data as of December 31, 2019, the market capitalization of a small cap company would be below $6,482 million. This threshold will change due to market conditions.

The U.S. Small Cap Value 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. Small Cap Value 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, and options thereon, 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. Small Cap Value 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, as well as 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. Small Cap Value Portfolio may lose money and there may be a delay in recovering the loaned securities. The U.S. Small Cap Value 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.

Operational Risk: Operational risks include human error, changes in personnel, system changes, faults in communication, and failures in systems, technology, or processes. Various operational events or circumstances are outside the Advisor’s control, including instances at third parties. The Portfolio and the Advisor seek to reduce these operational risks through controls and procedures. However, these measures do not address every possible risk and may be inadequate to address these risks.

Cyber Security Risk: The U.S. Small Cap Value 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. Small Cap Value 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. Small Cap Value 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. Small Cap Value 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. Small Cap Value 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. Small Cap Value 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. Small Cap Value 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. Small Cap Value Portfolio’s past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading] rr_BarChartHeading U.S. Small Cap Value Portfolio Institutional Class Shares—Total Returns
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2010-December  2019

Highest Quarter    Lowest Quarter
18.83% (10/10–12/10)
    -24.07% (7/11–9/11)
Performance Table Heading rr_PerformanceTableHeading Annualized Returns (%) Periods ending December 31, 2019
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate The after-tax returns presented in the table for the U.S. Small Cap Value 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. SMALL CAP VALUE PORTFOLIO | INSTITUTIONAL CLASS  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fee rr_ManagementFeesOverAssets 0.48% [1]
Other Expenses rr_OtherExpensesOverAssets 0.03% [1]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.51% [1]
1 Year rr_ExpenseExampleYear01 $ 52
3 Years rr_ExpenseExampleYear03 164
5 Years rr_ExpenseExampleYear05 285
10 Years rr_ExpenseExampleYear10 640
1 Year rr_ExpenseExampleNoRedemptionYear01 52
3 Years rr_ExpenseExampleNoRedemptionYear03 164
5 Years rr_ExpenseExampleNoRedemptionYear05 285
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 640
2010 rr_AnnualReturn2010 30.90%
2011 rr_AnnualReturn2011 (7.55%)
2012 rr_AnnualReturn2012 21.72%
2013 rr_AnnualReturn2013 42.38%
2014 rr_AnnualReturn2014 3.48%
2015 rr_AnnualReturn2015 (7.81%)
2016 rr_AnnualReturn2016 28.26%
2017 rr_AnnualReturn2017 7.21%
2018 rr_AnnualReturn2018 (15.13%)
2019 rr_AnnualReturn2019 18.12%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Dec. 31, 2010
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 18.83%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2011
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (24.07%)
1 Year rr_AverageAnnualReturnYear01 18.12%
5 Years rr_AverageAnnualReturnYear05 4.91%
10 Years rr_AverageAnnualReturnYear10 10.68%
U.S. SMALL CAP VALUE PORTFOLIO | Return After Taxes on Distributions | INSTITUTIONAL CLASS  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 17.33%
5 Years rr_AverageAnnualReturnYear05 3.66%
10 Years rr_AverageAnnualReturnYear10 9.60%
U.S. SMALL CAP VALUE PORTFOLIO | Return After Taxes on Distributions and Sale of Portfolio Shares | INSTITUTIONAL CLASS  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 11.27%
5 Years rr_AverageAnnualReturnYear05 3.71%
10 Years rr_AverageAnnualReturnYear10 8.72%
U.S. SMALL CAP VALUE PORTFOLIO | Russell 2000® Value Index (reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 22.39%
5 Years rr_AverageAnnualReturnYear05 6.99%
10 Years rr_AverageAnnualReturnYear10 10.56%
[1] The “Management Fee” and “Total Annual Fund Operating Expenses” have been adjusted to reflect the decrease in the management fee payable by the Portfolio from 0.50% to 0.48% effective as of February 28, 2020.