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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName T. Rowe Price Index Trust, Inc.
Prospectus Date rr_ProspectusDate Mar. 01, 2020
T. Rowe Price Small-Cap Index Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading T. ROWE PRICE Small-Cap Index Fund Investor Class I Class Z Class SUMMARY
Objective [Heading] rr_ObjectiveHeading Investment Objective(s)
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The fund seeks to track the performance of a benchmark index that measures the investment return of small-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, hold, and sell shares of the fund.
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Fees and Expenses of the Fund 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 April 30, 2022
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 17.3% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 17.30%
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 any current expense limitation arrangement remains in place for the period noted in the table above; 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 Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock The fund attempts to track the investment return of small-capitalization U.S. stocks by seeking to match the performance of the Russell 2000® Index (“benchmark 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 small-cap stocks and in stocks that are included in its benchmark index. The fund defines small-cap stocks as those whose market capitalization, at the time of purchase, falls within the range of its benchmark index. As of December 31, 2019, the market capitalization range for the benchmark index was approximately $12.7 million to $8.3 billion. T. Rowe Price compares the composition of the fund to that of the 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 Russell 2000® Index, which is constructed by the Russell Investment Group, is a well-known small-cap stock index that is designed to measure the performance of the small-cap segment of the U.S. equity universe. It includes approximately 2,000 small-cap U.S. securities based on a combination of their market capitalization and index membership, without consideration to a growth or value style. Some of the securities included in the benchmark index may be considered micro-cap securities. The benchmark index is constructed to provide an unbiased small-cap barometer and is reconstituted periodically to ensure larger stocks do not distort the performance and characteristics of the small-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 fund assets exposed to common stocks represented in the fund’s benchmark index, or as an efficient means of gaining small-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 fund, there is no guarantee that the fund will achieve its objective(s). 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, which may be even greater during periods of market disruption or volatility, are summarized as follows.

Index investing 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 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.

Stock investing 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 stocks held by the fund may decline due to general weakness or volatility in the stock markets in which the fund invests or because of factors that affect a particular company or industry.

Small- and micro-cap stocks Investments in securities issued by small-cap companies and micro-cap companies are likely to be more volatile than investments in securities issued by larger companies. Small-cap companies often have less experienced management, narrower product lines, more limited financial resources, and less publicly available information than larger companies. In addition, smaller companies are typically more sensitive to changes in overall economic conditions and their securities may be difficult to trade. The risks of small-cap investing are heightened for investments in micro-cap companies.

Futures The use of futures contracts potentially exposes the fund to greater volatility than directly purchasing securities in the index, including possible illiquidity of the futures markets, contract prices that can be volatile and imperfectly correlated to movements in underlying security values, and potential losses in excess of the fund’s initial investment.
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. 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. 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 troweprice.com
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 SMALL-CAP INDEX FUND Calendar Year Returns
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
  Quarter Ended    Total Return  Quarter Ended     Total Return
Best Quarter             3/31/19             14.52%      Worst Quarter            12/31/18            -20.26%
Performance Table Heading rr_PerformanceTableHeading Average Annual Total Returns Periods ended December 31, 2019
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 Small-Cap Index Fund | Investor Class  
Risk/Return: rr_RiskReturnAbstract  
Management fees rr_ManagementFeesOverAssets 0.14%
Other expenses rr_OtherExpensesOverAssets 3.79%
Total annual fund operating expenses rr_ExpensesOverAssets 3.93%
Fee waiver/expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (3.59%) [1]
Total annual fund operating expenses after fee waiver/expense reimbursement rr_NetExpensesOverAssets 0.34% [1]
1 year rr_ExpenseExampleYear01 $ 35
3 years rr_ExpenseExampleYear03 455
5 years rr_ExpenseExampleYear05 1,338
10 years rr_ExpenseExampleYear10 $ 3,631
2016 rr_AnnualReturn2016 21.05%
2017 rr_AnnualReturn2017 14.51%
2018 rr_AnnualReturn2018 (11.25%)
2019 rr_AnnualReturn2019 25.31%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2019
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 14.52%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2018
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (20.26%)
1 Year rr_AverageAnnualReturnYear01 25.31%
Since inception rr_AverageAnnualReturnSinceInception 11.04%
Inception date rr_AverageAnnualReturnInceptionDate Dec. 09, 2015
T. Rowe Price Small-Cap Index Fund | I Class  
Risk/Return: rr_RiskReturnAbstract  
Management fees rr_ManagementFeesOverAssets 0.14%
Other expenses rr_OtherExpensesOverAssets 3.57%
Total annual fund operating expenses rr_ExpensesOverAssets 3.71%
Fee waiver/expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (3.52%) [2]
Total annual fund operating expenses after fee waiver/expense reimbursement rr_NetExpensesOverAssets 0.19% [2]
1 year rr_ExpenseExampleYear01 $ 19
3 years rr_ExpenseExampleYear03 401
5 years rr_ExpenseExampleYear05 1,239
10 years rr_ExpenseExampleYear10 $ 3,430
1 Year rr_AverageAnnualReturnYear01 25.60%
Since inception rr_AverageAnnualReturnSinceInception 11.21%
Inception date rr_AverageAnnualReturnInceptionDate Dec. 09, 2015
T. Rowe Price Small-Cap Index Fund | Z Class  
Risk/Return: rr_RiskReturnAbstract  
Management fees rr_ManagementFeesOverAssets 0.14%
Other expenses rr_OtherExpensesOverAssets 3.57%
Total annual fund operating expenses rr_ExpensesOverAssets 3.71%
Fee waiver/expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (3.71%) [3]
Total annual fund operating expenses after fee waiver/expense reimbursement rr_NetExpensesOverAssets none [3]
1 year rr_ExpenseExampleYear01 none
3 years rr_ExpenseExampleYear03 none
5 years rr_ExpenseExampleYear05 none
10 years rr_ExpenseExampleYear10 none
1 Year rr_AverageAnnualReturnYear01
Since inception rr_AverageAnnualReturnSinceInception
Inception date rr_AverageAnnualReturnInceptionDate Mar. 16, 2020
T. Rowe Price Small-Cap Index Fund | Returns after taxes on distributions | Investor Class  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 23.90%
Since inception rr_AverageAnnualReturnSinceInception 9.35%
Inception date rr_AverageAnnualReturnInceptionDate Dec. 09, 2015
T. Rowe Price Small-Cap Index Fund | Returns after taxes on distributions and sale of fund shares | Investor Class  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 15.82%
Since inception rr_AverageAnnualReturnSinceInception 8.31%
Inception date rr_AverageAnnualReturnInceptionDate Dec. 09, 2015
T. Rowe Price Small-Cap Index Fund | Russell 2000® Index (reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 25.52%
Since inception rr_AverageAnnualReturnSinceInception 11.26% [4]
Inception date rr_AverageAnnualReturnInceptionDate Dec. 09, 2015
[1] T. Rowe Price Associates, Inc., has contractually agreed (through April 30, 2022) 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 exceed 0.34% of the class' average daily net assets. The agreement may only be terminated at any time after April 30, 2022, with approval by the fund’s Board of Directors. Fees waived and expenses paid under this agreement (and a previous limitation of 0.34%) are subject to reimbursement to T. Rowe Price Associates, Inc., by the fund whenever the class’ expense ratio is below 0.34%. 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.
[2] T. Rowe Price Associates, Inc., has contractually agreed (through April 30, 2022) 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 only be terminated at any time after April 30, 2022, 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.
[3] T. Rowe Price Associates, Inc., has contractually agreed to waive and/or bear all of the Z Class’ expenses (excluding interest; expenses related to borrowings, taxes, and brokerage; nonrecurring, extraordinary expenses; and acquired fund fees and expenses) in their entirety. T. Rowe Price Associates, Inc. expects this fee waiver and/or expense reimbursement arrangement to remain in place indefinitely, and the agreement may only be amended or terminated with approval by the fund’s Board of Directors.
[4] Return since 12/9/15.