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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2017
VIT Inflation-Protected Securities Portfolio  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading VIT Inflation-Protected Securities Portfolio
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the VIT Inflation-Protected Securities Portfolio (the “Inflation-Protected Portfolio” or “Portfolio”) is to provide inflation protection and earn current income consistent with inflation-protected securities.
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 Inflation-Protected Portfolio. The expenses in the table do not include any fees or charges imposed by the variable insurance contract. If such fees and charges were included, the expenses would be higher.
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 Inflation-Protected 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 Inflation-Protected Portfolio’s performance. During the most recent fiscal year, the Inflation-Protected Portfolio’s portfolio turnover rate was 21% of the average value of its investment portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 21.00%
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 Inflation-Protected Portfolio with the cost of investing in other mutual funds. The Example does not include any fees or charges imposed by the variable insurance contract and if such fees were included, expenses would be higher. 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. The costs for the Portfolio reflect the net expenses of the Portfolio that result from the contractual expense waiver and assumption in the first year only. 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 Inflation-Protected Portfolio seeks its investment objective by investing in a universe of inflation-protected securities that are structured to provide returns linked to the rate of inflation over the long-term. The Inflation-Protected Portfolio ordinarily invests in inflation-protected securities issued by the U.S. Government and its agencies and instrumentalities and the credit quality of such inflation-protected securities will be that of such applicable U.S. government, agency or instrumentality issuer.

As a non-fundamental policy, under normal circumstances, the Portfolio will invest at least 80% of its net assets in inflation-protected securities. Inflation-protected securities (also known as inflation-indexed securities) are securities whose principal and/or interest payments are adjusted for inflation, unlike conventional debt securities that make fixed principal and interest payments. Inflation-protected securities include Treasury Inflation-Protected Securities (“TIPS”), which are securities issued by the U.S. Treasury. The principal value of TIPS is adjusted for inflation (payable at maturity) and the semi-annual interest payments by TIPS equal a fixed percentage of the inflation-adjusted principal amount. These inflation adjustments are based upon the Consumer Price Index for Urban Consumers (CPI-U). The original principal value of TIPS is guaranteed, even during periods of deflation. At maturity, TIPS are redeemed at the greater of their inflation-adjusted principal or par amount at original issue. Other types of inflation-protected securities may use other methods to adjust for inflation and other measures of inflation. In addition, inflation-protected securities issued by entities other than the U.S. Treasury may not provide a guarantee of principal value at maturity.

Generally, the Inflation-Protected Portfolio will purchase inflation-protected securities with maturities of between five and twenty years from the date of settlement, although it is anticipated that, at times, the Portfolio will purchase securities outside of this range. The Portfolio ordinarily will have an average weighted maturity, based upon market values, of between three to twelve years.

The Inflation-Protected Portfolio is authorized to invest more than 25% of its total assets in Treasury bonds, bills and notes and obligations of U.S. government agencies and instrumentalities. The Portfolio will not shift the maturity of its investments in anticipation of interest rate movements.

The Inflation-Protected Portfolio may purchase or sell futures contracts and options on futures contracts, 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 Inflation-Protected 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 Inflation-Protected Portfolio will fluctuate, there is the risk that you will lose money. The following is a description of principal risks of investing in the Inflation-Protected Portfolio.

Market Risk: Even a long-term investment approach cannot guarantee a profit. Economic, political, and issuer-specific events will cause the value of securities, and the Portfolio that owns them, to rise or fall.

Interest Rate Risk: Fixed income securities are subject to interest rate risk because the prices of fixed income securities tend to move in the opposite direction of interest rates. When interest rates rise, fixed income security prices fall. When interest rates fall, fixed income security prices rise. In general, fixed income securities with longer maturities are more sensitive to changes in interest rates.

Inflation-Protected Securities Interest Rate Risk: Inflation-protected securities may react differently from other fixed income securities to changes in interest rates. Because interest rates on inflation-protected securities are adjusted for inflation, the values of these securities are not materially affected by inflation expectations. Therefore, the value of inflation-protected securities are anticipated to change in response to changes in “real” interest rates, which represent nominal (stated) interest rates reduced by the expected impact of inflation. Generally, the value of an inflation-protected security will fall when real interest rates rise and will rise when real interest rates fall.

Credit Risk: Credit risk is the risk that the issuer of a security may be unable to make interest payments and/or repay principal when due. A downgrade to an issuer’s credit rating or a perceived change in an issuer’s financial strength may affect a security’s value, and thus, impact the Inflation-Protected Portfolio’s performance. Government agency obligations have different levels of credit support and, therefore, different degrees of credit risk. Securities issued by agencies and instrumentalities of the U.S. Government that are supported by the full faith and credit of the United States, such as the Federal Housing Administration and Ginnie Mae, present little credit risk. Other securities issued by agencies and instrumentalities sponsored by the U.S. Government, that are supported only by the issuer’s right to borrow from the U.S. Treasury, subject to certain limitations, and securities issued by agencies and instrumentalities sponsored by the U.S. Government that are sponsored by the credit of the issuing agencies, such as Freddie Mac and Fannie Mae, are subject to a greater degree of credit risk. U.S. government agency securities issued or guaranteed by the credit of the agency may still involve a risk of non-payment of principal and/or interest.

Risks of Investing for Inflation Protection: Because the interest and/or principal payments on an inflation- protected security are adjusted periodically for changes in inflation, the income distributed by the Inflation-Protected Portfolio may be irregular. Although the U.S. Treasury guarantees to pay at least the original face value of any inflation-protected securities the Treasury issues, other issuers may not offer the same guarantee. Also, inflation-protected securities, including those issued by the U.S. Treasury, are not protected against deflation. As a result, in a period of deflation, the principal and income of inflation-protected securities held by the Portfolio will decline and the Portfolio may suffer a loss during such periods. While inflation-protected securities are expected to be protected from long-term inflationary trends, short-term increases in inflation may lead to a decline in the Portfolio’s value. For example, if interest rates rise due to reasons other than inflation, the Portfolio’s investment in these securities may not be protected to the extent that the increase is not reflected in the securities’ inflation measures. In addition, positive adjustments to principal generally will result in taxable income to the Portfolio at the time of such adjustments (which generally would be distributed by the Portfolio as part of its taxable dividends), even though the principal amount is not paid until maturity. The current market value of inflation-protected securities is not guaranteed and will fluctuate.

Income Risk: Income risk is the risk that falling interest rates will cause the Inflation-Protected Portfolio’s income to decline because, among other reasons, the proceeds from maturing short-term securities in its portfolio may be reinvested in lower-yielding securities.

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 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.

Liquidity Risk: Liquidity risk exists when particular portfolio investments are difficult to purchase or sell. To the extent that the Inflation-Protected Portfolio holds illiquid investments, the Portfolio’s performance may be reduced due to an inability to sell the investments at opportune prices or times. Liquid portfolio investments may become illiquid or less liquid after purchase by the Inflation-Protected Portfolio due to low trading volume, adverse investor perceptions and/or other market developments. Liquidity risk includes the risk that the Inflation-Protected Portfolio will experience significant net redemptions at a time when it cannot find willing buyers for its portfolio securities or can only sell its portfolio securities at a material loss. Liquidity risk can be more pronounced in periods of market turmoil.

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 Inflation-Protected Portfolio may lose money and there may be a delay in recovering the loaned securities. The 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 Inflation-Protected 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 Inflation-Protected 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 Inflation-Protected 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 Inflation-Protected 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 Inflation-Protected 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 Inflation-Protected 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 Inflation-Protected 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 Inflation-Protected 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 Inflation-Protected Portfolio’s past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading] rr_BarChartHeading Inflation-Protected Portfolio—Total Returns
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2016-December 2016

Highest Quarter    Lowest Quarter
4.89% (1/16–3/16)
    -2.98% (10/16–12/16)
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 Inflation-Protected 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.
VIT Inflation-Protected Securities Portfolio | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
Management Fee rr_ManagementFeesOverAssets 0.10%
Other Expenses rr_OtherExpensesOverAssets 0.09%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.19%
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 60
5 Years rr_ExpenseExampleYear05 106
10 Years rr_ExpenseExampleYear10 242
1 Year rr_ExpenseExampleNoRedemptionYear01 18
3 Years rr_ExpenseExampleNoRedemptionYear03 60
5 Years rr_ExpenseExampleNoRedemptionYear05 106
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 242
2016 rr_AnnualReturn2016 4.49%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2016
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 4.89%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2016
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (2.98%)
1 Year rr_AverageAnnualReturnYear01 4.49%
Since Inception rr_AverageAnnualReturnSinceInception 0.48%
Inception Date rr_AverageAnnualReturnInceptionDate May 29, 2015
VIT Inflation-Protected Securities Portfolio | Return After Taxes on Distributions | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 3.90%
Since Inception rr_AverageAnnualReturnSinceInception 0.03%
Inception Date rr_AverageAnnualReturnInceptionDate May 29, 2015
VIT Inflation-Protected Securities Portfolio | Return After Taxes on Distributions and Sale of Portfolio Shares | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 2.54%
Since Inception rr_AverageAnnualReturnSinceInception 0.16%
Inception Date rr_AverageAnnualReturnInceptionDate May 29, 2015
VIT Inflation-Protected Securities Portfolio | Bloomberg Barclays U.S. TIPS Index (reflects no deduction for fees, expenses, or taxes on sales)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 4.68%
Since Inception rr_AverageAnnualReturnSinceInception 1.14%
[1] The Advisor has agreed to waive certain fees and in certain instances, assume certain expenses of the Inflation-Protected 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 and/or expense assumption.