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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName T. Rowe Price Index Trust, Inc.
Prospectus Date rr_ProspectusDate May 01, 2019
T. Rowe Price Mid-Cap Index Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading T. ROWE PRICE<br/><br/> Mid-Cap Index Fund<br/><br/> Investor Class<br/><br/> I Class<br/><br/> SUMMARY
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The fund seeks to track the performance of a benchmark index that measures the investment return of mid-capitalization U.S. stocks.
Expense [Heading] rr_ExpenseHeading Fees and Expenses
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock This table describes the fees and expenses that you may pay if you buy and hold shares of the fund.
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Fees and Expenses of the Fund<br/><br/> <b>Annual fund operating expenses<br/> (expenses that you pay each year as a <br/>percentage of the value of your investment)</b>
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination April 30, 2020
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading Portfolio Turnover
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when the fund’s shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the fund’s performance. During the most recent fiscal year, the fund’s portfolio turnover rate was 32.3% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 32.30%
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the fund for the time periods indicated and then redeem all of your shares at the end of those periods, that your investment has a 5% return each year, and that the fund’s operating expenses remain the same. The example also assumes that an expense limitation arrangement currently in place is not renewed; therefore, the figures have been adjusted to reflect fee waivers or expense reimbursements only in the periods for which the expense limitation arrangement is expected to continue. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading Investments, Risks, and Performance<br/> <br/>Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock The fund attempts to track the investment return of mid-capitalization U.S. stocks by seeking to match the performance of the Russell Select Midcap Index. The fund seeks to employ a full replication strategy, which involves investing substantially all of its assets in all of the stocks in the benchmark index and seeking to maintain holdings of each stock in proportion to its weight in the index.

Under normal conditions, the fund invests at least 80% of its net assets (including any borrowings for investment purposes) in mid-cap stocks and in stocks that are included in its benchmark index. The fund defines mid-cap stocks as those whose market capitalization, at the time of purchase, falls within the range of its benchmark index. As of December 31, 2018, the market capitalization range for the benchmark index was approximately $473.5 million to $57.2 billion. T. Rowe Price compares the composition of the fund to that of the benchmark index. If a material misweighting develops, the portfolio manager seeks to rebalance the portfolio in an effort to realign the fund with the benchmark index.

The benchmark index, which is constructed by the Russell Investment Group, is a mid-cap stock index designed to measure the performance of the mid-cap segment of the U.S. equity universe. It includes approximately 500 mid-cap U.S. securities based on market capitalization without consideration to a growth or value style. The benchmark index is generally constructed by including all of the stocks in the Russell 1000 Index except for those also represented in the S&P 500 Index, and is reconstituted periodically to ensure larger or smaller stocks do not distort the performance and characteristics of the mid-cap opportunity set.

While most assets will be invested in common stocks, the fund may also purchase stock index futures contracts. Futures would typically be used to reduce cash balances in the fund and increase the level of the fund’s assets exposed to common stocks represented in the fund’s benchmark index, or as an efficient means of gaining mid-cap stock exposure.

While there is no guarantee, the correlation between the fund and its benchmark index is expected to be at least 0.95. A correlation of 1.00 indicates that the returns of the fund and the index will always move in the same direction (but not necessarily by the same amount). A correlation of 0.00 would mean price movements in the fund are unrelated to price movements in the index.

The fund may sell securities to better align its portfolio with the characteristics of its benchmark index or to satisfy redemption requests. The fund will generally seek to sell securities that have been removed from the benchmark index within a reasonable timeframe taking into consideration market conditions.
Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock As with any mutual fund, there is no guarantee that the fund will achieve its objective. The fund’s share price fluctuates, which means you could lose money by investing in the fund. The principal risks of investing in this fund are summarized as follows:

Risks of U.S. stock investing Common stocks generally fluctuate in value more than bonds and may decline significantly over short time periods. There is a chance that stock prices overall will decline because stock markets tend to move in cycles, with periods of rising and falling prices. The value of a stock in which the fund invests may decline due to general weakness in the U.S. stock market, such as when the U.S. financial markets decline, or because of factors that affect a particular company or industry.

Mid-cap stock risks Investing primarily in issuers within the same market capitalization category carries the risk that the category may be out of favor due to current market conditions or investor sentiment. Because the fund invests primarily in securities issued by mid-cap companies, it is likely to be more volatile than a fund that focuses on securities issued by large companies. Medium-sized companies may have less seasoned management, narrower product lines, and less capital reserves and liquidity than larger companies, and are therefore more sensitive to economic, market, and industry changes.

Index investing risks Because the fund is passively managed and seeks to match the performance of its benchmark index, holdings are generally not reallocated based on changes in market conditions or outlook for a specific security, industry, or market sector. As a result, the fund’s performance may lag the performance of actively managed funds.

Tracking error risks The returns of the fund are expected to be slightly below the returns of its benchmark index (referred to as “tracking error”) because the fund incurs fees and transaction expenses while the index has no fees or expenses. The risk of tracking error is increased to the extent the fund is unable to fully replicate its benchmark index, which could result from changes in the composition of the index or the timing of purchases and redemptions of fund shares.

Futures risks The fund’s use of stock index futures exposes it to potential volatility and losses in excess of direct investments in the contract’s underlying assets. The values of the fund’s positions in stock index futures tend to fluctuate in response to changes in the value of the underlying index, which exposes the fund to the risk that the underlying index will not move in a direction that is favorable to the fund. While the value of a stock index futures contract tends to correlate with the value of the underlying index, differences between the futures market and the value of the underlying index may result in an imperfect correlation. Since losses could result from market movement, the fund may need to sell other portfolio securities at disadvantageous times in order to meet daily margin requirements. The futures markets may experience reduced liquidity, which could result in losses to the fund and cause the fund to be unable to settle its futures positions.
Risk Lose Money [Text] rr_RiskLoseMoney The fund’s share price fluctuates, which means you could lose money by investing in the fund.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The following performance information provides some indication of the risks of investing in the fund. The fund’s performance information represents only past performance (before and after taxes) and is not necessarily an indication of future results.

The following bar chart illustrates how much returns can differ from year to year by showing calendar year returns and the best and worst calendar quarter returns during those years for the fund’s Investor Class. Returns for other share classes vary since they have different expenses.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The following performance information provides some indication of the risks of investing in the fund.<br/><br/> The following bar chart illustrates how much returns can differ from year to year by showing calendar year returns and the best and worst calendar quarter returns during those years for the fund's Investor Class. Returns for other share classes vary since they have different expenses.<br/><br/> The following table shows the average annual total returns for each class of the fund that has been in operation for at least one full calendar year, and also compares the returns with the returns of a relevant broad-based market index, as well as with the returns of one or more comparative indexes that have investment characteristics similar to those of the fund, if applicable.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress <b>troweprice.com</b>
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The fund’s performance information represents only past performance (before and after taxes) and is not necessarily an indication of future results.
Bar Chart [Heading] rr_BarChartHeading <b>MID-CAP INDEX FUND<br/> Calendar Year Returns</b>
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
  Quarter Ended    Total Return  Quarter Ended     Total Return
Best Quarter             12/31/17             6.12%      Worst Quarter            12/31/18            -16.13%
Performance Table Heading rr_PerformanceTableHeading <b>Average Annual Total Returns<br/><br/> Periods ended<br/> December 31, 2018<br/></b>
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate After-tax returns 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 Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their fund shares through tax-deferred arrangements, such as a 401(k) account or an IRA.
Performance Table One Class of after Tax Shown [Text] rr_PerformanceTableOneClassOfAfterTaxShown After-tax returns are shown only for the Investor Class and will differ for other share classes.
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock The following table shows the average annual total returns for each class of the fund that has been in operation for at least one full calendar year, and also compares the returns with the returns of a relevant broad-based market index, as well as with the returns of one or more comparative indexes that have investment characteristics similar to those of the fund, if applicable.

In addition, the table shows hypothetical after-tax returns to demonstrate how taxes paid by a shareholder may influence returns. After-tax returns 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. After-tax returns shown are not relevant to investors who hold their fund shares through tax-deferred arrangements, such as a 401(k) account or an IRA. After-tax returns are shown only for the Investor Class and will differ for other share classes.
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock Updated performance information is available through troweprice.com.
T. Rowe Price Mid-Cap Index Fund | Investor Class  
Risk/Return: rr_RiskReturnAbstract  
Management fees rr_ManagementFeesOverAssets 0.12%
Other expenses rr_OtherExpensesOverAssets 3.28%
Total annual fund operating expenses rr_ExpensesOverAssets 3.40%
Fee waiver/expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (3.15%) [1],[2]
Total annual fund operating expenses after fee waiver/expense reimbursement rr_NetExpensesOverAssets 0.25% [2]
1 year rr_ExpenseExampleYear01 $ 26
3 years rr_ExpenseExampleYear03 749
5 years rr_ExpenseExampleYear05 1,496
10 years rr_ExpenseExampleYear10 $ 3,471
2016 rr_AnnualReturn2016 13.33%
2017 rr_AnnualReturn2017 20.88%
2018 rr_AnnualReturn2018 (8.25%)
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel <b>Best Quarter</b>
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Dec. 31, 2017
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 6.12%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel <b>Worst Quarter</b>
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2018
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (16.13%)
1 Year rr_AverageAnnualReturnYear01 (8.25%)
Since inception rr_AverageAnnualReturnSinceInception 7.76%
Inception date rr_AverageAnnualReturnInceptionDate Dec. 09, 2015
T. Rowe Price Mid-Cap Index Fund | I Class  
Risk/Return: rr_RiskReturnAbstract  
Management fees rr_ManagementFeesOverAssets 0.12%
Other expenses rr_OtherExpensesOverAssets 3.22% [3]
Total annual fund operating expenses rr_ExpensesOverAssets 3.34%
Fee waiver/expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (3.17%) [1],[3]
Total annual fund operating expenses after fee waiver/expense reimbursement rr_NetExpensesOverAssets 0.17% [3]
1 year rr_ExpenseExampleYear01 $ 17
3 years rr_ExpenseExampleYear03 729
5 years rr_ExpenseExampleYear05 1,465
10 years rr_ExpenseExampleYear10 $ 3,414
1 Year rr_AverageAnnualReturnYear01 (8.17%)
Since inception rr_AverageAnnualReturnSinceInception 7.88%
Inception date rr_AverageAnnualReturnInceptionDate Dec. 09, 2015
T. Rowe Price Mid-Cap Index Fund | Returns after taxes on distributions | Investor Class  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 (9.72%)
Since inception rr_AverageAnnualReturnSinceInception 6.46%
Inception date rr_AverageAnnualReturnInceptionDate Dec. 09, 2015
T. Rowe Price Mid-Cap Index Fund | Returns after taxes on distributions and sale of fund shares | Investor Class  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 (3.96%)
Since inception rr_AverageAnnualReturnSinceInception 5.83%
Inception date rr_AverageAnnualReturnInceptionDate Dec. 09, 2015
T. Rowe Price Mid-Cap Index Fund | Russell Select Mid Cap Index (reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 (8.17%)
Since inception rr_AverageAnnualReturnSinceInception 7.84% [4]
Inception date rr_AverageAnnualReturnInceptionDate Dec. 09, 2015
[1] As a result of class-specific expense limitations, T. Rowe Price Associates, Inc. waived fund-level expenses ratably across all classes.
[2] T. Rowe Price Associates, Inc., has agreed (through April 30, 2020) to waive its fees and/or bear any expenses (excluding interest; expenses related to borrowings, taxes, and brokerage; nonrecurring, extraordinary expenses; and acquired fund fees and expenses) that would cause the class' ratio of expenses to average daily net assets to exceed 0.32%. The agreement may be terminated at any time beyond April 30, 2020, with approval by the fund's Board of Directors. Fees waived and expenses paid under this agreement (and a previous limitation of 0.32%) are subject to reimbursement to T. Rowe Price Associates, Inc., by the fund whenever the class' expense ratio is below 0.32%. However, no reimbursement will be made more than three years from the date such amounts were initially waived or reimbursed. The fund may only make repayments to T. Rowe Price Associates, Inc., if such repayment does not cause the class' expense ratio (after the repayment is taken into account) to exceed the lesser of: (1) the expense limitation in place at the time such amounts were waived; or (2) the class' current expense limitation.
[3] T. Rowe Price Associates, Inc., has agreed (through April 30, 2020) to pay the operating expenses of the fund's I Class excluding management fees; interest; expenses related to borrowings, taxes, and brokerage; nonrecurring, extraordinary expenses; and acquired fund fees and expenses ("I Class Operating Expenses"), to the extent the I Class Operating Expenses exceed 0.05% of the class' average daily net assets. The agreement may be terminated at any time beyond April 30, 2020, with approval by the fund's Board of Directors. Any expenses paid under this agreement (and a previous limitation of 0.05%) are subject to reimbursement to T. Rowe Price Associates, Inc., by the fund whenever the fund's I Class Operating Expenses are below 0.05%. However, no reimbursement will be made more than three years from the date such amounts were initially waived or reimbursed. The fund may only make repayments to T. Rowe Price Associates, Inc., if such repayment does not cause the I Class Operating Expenses (after the repayment is taken into account) to exceed the lesser of: (1) the limitation on I Class Operating Expenses in place at the time such amounts were waived; or (2) the current expense limitation on I Class Operating Expenses.
[4] Return since 12/9/15.