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Class F Prospectus | SIMT Multi-Strategy Alternative Fund  
Risk/Return: rr_RiskReturnAbstract  
Supplement [Text Block] ck0000804239_SupplementTextBlock

SEI INSTITUTIONAL MANAGED TRUST


Core Fixed Income Fund
Multi-Strategy Alternative Fund
Long/Short Alternative Fund
(the "Funds")


Supplement Dated September 26, 2018
to the Class F Prospectus dated January 31, 2018, as amended on April 19, 2018, May 1, 2018,
May 3, 2018, July 9, 2018 and August 3, 2018


This Supplement provides new and additional information beyond that contained in the Prospectus and should be read in conjunction with such Prospectus.


The Prospectus is hereby amended and supplemented to reflect the following changes to the Funds.
Risk/Return [Heading] rr_RiskReturnHeading SIMT Multi-Strategy Alternative Fund
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock

Changes to the Fees and Expenses of the Multi-Strategy Alternative Fund


In the Fund Summary for the Multi-Strategy Alternative Fund, under the heading "Fees and Expenses" (but excluding the sub-heading titled "Portfolio Turnover"), the text and tables are hereby deleted and replaced with the following:


This table describes the fees and expenses that you may pay if you buy and hold Fund shares.
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)
Expenses Restated to Reflect Current [Text] rr_ExpensesRestatedToReflectCurrent Other Expenses have been restated to reflect estimated fees and expenses for the remainder of the current fiscal year and the upcoming fiscal year.
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text] rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees The Fund is anticipated to incur AFFE during the current fiscal year, and therefore the operating expenses in this fee table will not correlate to the expense ratio in the Fund's financial statements (or the "Financial Highlights" section in the prospectus). The financial statements include only the direct operating expenses incurred by the Fund, not the indirect costs of investing in other investment companies.
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:
Expense Example Closing [Text Block] rr_ExpenseExampleClosingTextBlock

There are no other changes to the Fees and Expenses of the Multi-Strategy Alternative Fund.


PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE
Class F Prospectus | SIMT Multi-Strategy Alternative Fund | Class F  
Risk/Return: rr_RiskReturnAbstract  
Management Fees rr_ManagementFeesOverAssets 1.50%
Distribution (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Dividends on Shorts rr_Component2OtherExpensesOverAssets 0.35%
Remainder of Other Expenses rr_Component3OtherExpensesOverAssets 0.59%
Total Other Expenses rr_OtherExpensesOverAssets 0.94% [1]
Acquired Fund Fees and Expenses (AFFE) rr_AcquiredFundFeesAndExpensesOverAssets 0.53%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 2.97% [2]
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 300
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 918
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 1,562
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 3,290
[1] Other Expenses have been restated to reflect estimated fees and expenses for the remainder of the current fiscal year and the upcoming fiscal year.
[2] The Fund is anticipated to incur AFFE during the current fiscal year, and therefore the operating expenses in this fee table will not correlate to the expense ratio in the Fund's financial statements (or the "Financial Highlights" section in the prospectus). The financial statements include only the direct operating expenses incurred by the Fund, not the indirect costs of investing in other investment companies.