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Class F Prospectus | SIMT S&P 500 Index Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading SIMT S&P 500 Index Fund
Objective [Heading] rr_ObjectiveHeading Investment Goal
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

Investment results that correspond to the aggregate price and dividend performance of the securities in the S&P 500 Index.
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 Fund shares.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption 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 Jan. 31, 2021
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 Fund 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% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 17.00%
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. The Example also assumes that your investment has a 5% return each year and that the Fund's operating expenses remain the same.
Expense Example by, Year, Caption [Text] rr_ExpenseExampleByYearCaption 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 Fund invests substantially all of its assets (at least 80%) in securities listed in the Standard & Poor's 500 Composite Stock Price Index (S&P 500 Index), which is composed of approximately 500 leading U.S. publicly traded companies from a broad range of industries (mostly common stocks). The Fund's investment results are expected to correspond to the aggregate price and dividend performance of the S&P 500 Index before the fees and expenses of the Fund. The Fund generally gives the same weight to a given stock as the S&P 500 Index does.


In seeking to replicate the performance of the S&P 500 Index, the Fund may also invest in futures contracts, American Depositary Receipts (ADRs), real estate investment trusts (REITs) and exchange-traded funds (ETFs). The Fund may also invest a portion of its assets in securities of companies located in developed foreign countries and securities of small capitalization companies. The Fund's ability to replicate the performance of the S&P 500 Index will depend to some extent on the size and timing of cash flows into and out of the Fund, as well as on the level of the Fund's expenses. The Fund may use futures contracts to obtain exposure to the equity market during high volume periods of investment into the Fund. The Fund's sub-adviser (the Sub-Adviser) selects the Fund's securities under the general supervision of the Fund's adviser, SEI Investments Management Corporation (SIMC or the Adviser), but the Sub-Adviser makes no attempt to "manage" the Fund in the traditional sense (i.e., by using economic, market or financial analyses). Instead, the Sub-Adviser purchases a basket of securities that includes most of the companies in the S&P 500 Index. However, the Sub-Adviser may sell an investment if the merit of the investment has been substantially impaired by extraordinary events or adverse financial conditions.
Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock

Market Risk — The risk that the market value of a security may move up and down, sometimes rapidly and unpredictably. Market risk may affect a single issuer, an industry, a sector or the equity or bond market as a whole.


Large Capitalization Risk — The risk that larger, more established companies may be unable to respond quickly to new competitive challenges such as changes in technology and consumer tastes. Larger companies also may not be able to attain the high growth rates of successful smaller companies.


Investment Style Risk — The risk that the Fund's investment approach, which attempts to replicate the performance of the S&P 500 Index, may underperform other segments of the equity markets or the equity markets as a whole.


Tracking Error Risk — The risk that the Fund's performance may vary substantially from the performance of the benchmark index it tracks as a result of cash flows, Fund expenses, imperfect correlation between the Fund's investments and the benchmark and other factors.


Derivatives Risk — The Fund's use of futures contracts is subject to market risk, leverage risk, correlation risk and liquidity risk. Leverage risk and liquidity risk are described below. Market risk is the risk that the market value of an investment may move up and down, sometimes rapidly and unpredictably. Correlation risk is the risk that changes in the value of the derivative may not correlate perfectly with the underlying asset, rate or index. Each of these risks could cause the Fund to lose more than the principal amount invested in a derivative instrument. The Fund's use of derivatives may also increase the amount of taxes payable by shareholders. Both U.S. and non-U.S. regulators are in the process of adopting and implementing regulations governing derivatives markets, the ultimate impact of which remains unclear.


Depositary Receipts Risk — Depositary receipts, such as ADRs, are certificates evidencing ownership of shares of a foreign issuer that are issued by depositary banks and generally trade on an established market. Depositary receipts are subject to many of the risks associated with investing directly in foreign securities, including, among other things, political, social and economic developments abroad, currency movements and different legal, regulatory and tax environments.


Exchange-Traded Funds (ETFs) Risk — The risks of owning shares of an ETF generally reflect the risks of owning the underlying securities the ETF is designed to track, although lack of liquidity in an ETF could result in its value being more volatile than the underlying portfolio securities. When the Fund invests in an ETF, in addition to directly bearing the expenses associated with its own operations, it will bear a pro rata portion of the ETF's expenses.


Real Estate Investment Trusts (REITs) Risk — REITs are trusts that invest primarily in commercial real estate or real estate-related loans. The Fund's investments in REITs will be subject to the risks associated with the direct ownership of real estate. Risks commonly associated with the direct ownership of real estate include fluctuations in the value of underlying properties, defaults by borrowers or tenants, changes in interest rates and risks related to general or local economic conditions. Some REITs may have limited diversification and may be subject to risks inherent in financing a limited number of properties.


Liquidity Risk — The risk that certain securities may be difficult or impossible to sell at the time and the price that the Fund would like. The Fund may have to lower the price of the security, sell other securities instead or forego an investment opportunity, any of which could have a negative effect on Fund management or performance.


Leverage Risk — The Fund's use of derivatives may result in the Fund's total investment exposure substantially exceeding the value of its portfolio securities and the Fund's investment returns depending substantially on the performance of securities that the Fund may not directly own. The use of leverage can amplify the effects of market volatility on the Fund's share price and may also cause the Fund to liquidate portfolio positions when it would not be advantageous to do so in order to satisfy its obligations. The Fund's use of leverage may result in a heightened risk of investment loss.


Small Capitalization Risk — Smaller capitalization companies in which the Fund may invest may be more vulnerable to adverse business or economic events than larger, more established companies. In particular, small capitalization companies may have limited product lines, markets and financial resources and may depend upon a relatively small management group. Therefore, small capitalization stocks may be more volatile than those of larger companies. Small capitalization stocks may be traded over-the-counter or listed on an exchange.


Foreign Investment Risk — The risk that non-U.S. securities may be subject to additional risks due to, among other things, political, social and economic developments abroad, currency movements and different legal, regulatory and tax environments.


Currency Risk — As a result of the Fund's investments in securities or other investments denominated in, and/or receiving revenues in, foreign currencies, the Fund will be subject to currency risk. Currency risk is the risk that foreign currencies will decline in value relative to the U.S. dollar or, in the case of hedging positions, that the U.S. dollar will decline in value relative to the currency hedged. In either event, the dollar value of an investment in the Fund would be adversely affected. Currency exchange rates may fluctuate in response to, among other things, changes in interest rates, intervention (or failure to intervene) by U.S. or foreign governments, central banks or supranational entities, or by the imposition of currency controls or other political developments in the United States or abroad.


Investing in the Fund involves risk, and there is no guarantee that the Fund will achieve its investment goal. You could lose money on your investment in the Fund, just as you could with other investments. An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Risk Lose Money [Text] rr_RiskLoseMoney You could lose money on your investment in the Fund, just as you could with other investments.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution An investment in the Fund 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 Information
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock

The bar chart and the performance table below provide some indication of the risks of investing in the Fund by showing changes in the Fund's performance from year to year for the past ten calendar years and by showing how the Fund's average annual returns for 1, 5 and 10 years, and since the Fund's inception, compare with those of a broad measure of market performance. The Fund's past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. For current performance information, please call 1-800-DIAL-SEI.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The bar chart and the performance table below provide some indication of the risks of investing in the Fund by showing changes in the Fund's performance from year to year for the past ten calendar years and by showing how the Fund's average annual returns for 1, 5 and 10 years, and since the Fund's inception, compare with those of a broad measure of market performance.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone 1-800-DIAL-SEI
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The Fund's past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
Best Quarter: 15.90% (06/30/09)
Worst Quarter: -13.97% (09/30/11)




Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter:
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 15.90%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter:
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2011
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (13.97%)
Performance Table Heading rr_PerformanceTableHeading Average Annual Total Returns (for the periods ended December 31, 2018)
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees, expenses or taxes)
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 Your actual after-tax returns will depend on your 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 401(k) plans or individual retirement accounts.
Performance Table Explanation after Tax Higher rr_PerformanceTableExplanationAfterTaxHigher In the event of negative performance, the Fund's returns after taxes on distributions and sale of Fund shares are calculated assuming that an investor has sufficient capital gains of the same character from other investments to offset any capital losses from the sale of Fund shares. As a result, the Fund's returns after taxes on distributions and sale of Fund shares may exceed the Fund's returns before taxes and/or returns after taxes on distributions.
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock

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. Your actual after-tax returns will depend on your 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 401(k) plans or individual retirement accounts. In the event of negative performance, the Fund's returns after taxes on distributions and sale of Fund shares are calculated assuming that an investor has sufficient capital gains of the same character from other investments to offset any capital losses from the sale of Fund shares. As a result, the Fund's returns after taxes on distributions and sale of Fund shares may exceed the Fund's returns before taxes and/or returns after taxes on distributions.
Class F Prospectus | SIMT S&P 500 Index Fund | S&P 500 Index Return (reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel S&P 500 Index Return (reflects no deduction for fees, expenses or taxes)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (4.38%) [1]
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 8.49% [1]
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 13.12% [1]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 8.16% [1]
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Feb. 28, 1996
Class F Prospectus | SIMT S&P 500 Index Fund | Class F  
Risk/Return: rr_RiskReturnAbstract  
Management Fees rr_ManagementFeesOverAssets 0.03%
Distribution (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.52%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.55%
Fee Waivers and Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.26%)
Total Annual Fund Operating Expenses Less Fee Waivers and Expense Reimbursements rr_NetExpensesOverAssets 0.29% [2]
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 30
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 122
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 254
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 638
Annual Return 2009 rr_AnnualReturn2009 26.21%
Annual Return 2010 rr_AnnualReturn2010 14.80%
Annual Return 2011 rr_AnnualReturn2011 1.77%
Annual Return 2012 rr_AnnualReturn2012 15.56%
Annual Return 2013 rr_AnnualReturn2013 31.81%
Annual Return 2014 rr_AnnualReturn2014 13.21%
Annual Return 2015 rr_AnnualReturn2015 1.05%
Annual Return 2016 rr_AnnualReturn2016 11.47%
Annual Return 2017 rr_AnnualReturn2017 21.31%
Annual Return 2018 rr_AnnualReturn2018 (4.63%)
Label rr_AverageAnnualReturnLabel Return Before Taxes
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (4.63%) [1]
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 8.09% [1]
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 12.73% [1]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 7.73% [1]
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Feb. 28, 1996
Class F Prospectus | SIMT S&P 500 Index Fund | Class F | After Taxes on Distributions  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (5.47%) [1]
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 6.88% [1]
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 11.85% [1]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 7.01% [1]
Class F Prospectus | SIMT S&P 500 Index Fund | Class F | After Taxes on Distributions and Sale of Fund Shares  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions and Sale of Fund Shares
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (2.29%) [1]
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 6.32% [1]
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 10.74% [1]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 6.54% [1]
[1] The Fund is the successor to SEI Index Funds' S&P 500 Index Fund (the Predecessor Fund). The Fund commenced operations on September 17, 2007. Accordingly, for periods prior to September 17, 2007, performance information is based on the performance of the Predecessor Fund.
[2] Effective January 31, 2018, the Fund's adviser, the Fund's administrator and/or the Fund's distributor have contractually agreed to waive fees and/or reimburse expenses in order to keep total ordinary operating expenses (exclusive of interest from borrowing, brokerage commissions, taxes, Trustee fees, acquired fund fees and expenses and other extraordinary expenses not incurred in the ordinary course of the Fund's business) from exceeding 0.29%. This fee waiver and reimbursement agreement shall remain in effect until January 31, 2021. The agreement may be amended or terminated only with the consent of the Board of Trustees.