Label	Element	Value
SIMT CORE FIXED INCOME FUND
Risk/Return:	rr_RiskReturnAbstract
Supplement [Text Block]	ck0000804239_SupplementTextBlock	SEI INSTITUTIONAL MANAGED TRUST Core Fixed Income Fund (the "Fund") Supplement dated April 1, 2014 to the Class I Shares Prospectus (the "Prospectus") dated January 31, 2014 This Supplement provides new and additional information beyond that contained in the Class I Shares Prospectus and should be read in conjunction with such Prospectus. The Prospectus is hereby amended and supplemented to reflect the following changes in the performance information of the Core Fixed Income Fund.
Risk/Return [Heading]	rr_RiskReturnHeading	SIMT CORE FIXED INCOME FUND
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Change in Performance Information for the Core Fixed Income Fund
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	In the table under the heading "Average Annual Total Returns (for the periods ended December 31, 2013)," in the "Performance Information" section of the Fund Summary for the Core Fixed Income Fund, the Barclays U.S. Aggregate Bond Index Return, Since Inception, is hereby deleted and replaced with 6.88%.
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	There are no other changes in the performance information of the Core Fixed Income Fund. PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE

If the Fund has annual returns for at least one calendar year, provide a table showing the Fund's (A) average annual total return; (B) average annual total return (after taxes on distributions); and (C) average annual total return (after taxes on distributions and redemption). A Money Market Fund should show only the returns described in clause (A) of the preceding sentence. All returns should be shown for 1-, 5-, and 10- calendar year periods ending on the date of the most recently completed calendar year (or for the life of the Fund, if shorter), but only for periods subsequent to the effective date of the Fund's registration statement. The table also should show the returns of an appropriate broad-based securities market index as defined in Instruction 5 to Item 22(b)(7) for the same periods. A Fund that has been in existence for more than 10 years also may include returns for the life of the Fund. A Money Market Fund may provide the Fund's 7-day yield ending on the date of the most recent calendar year or disclose a toll-free (or collect) telephone number that investors can use to obtain the Fund's current 7-day yield. For a Fund (other than a Money Market Fund or a Fund described in General Instruction C.3.(d)(iii)), provide the information in the following table with the specified captions AVERAGE ANNUAL TOTAL RETURNS (For the periods ended December 31, _____)

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