Label	Element	Value
SIMT Small Cap Fund
Risk/Return:	rr_RiskReturnAbstract
Supplement [Text Block]	ck0000804239_SupplementTextBlock	SEI INSTITUTIONAL MANAGED TRUST Small Cap 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 Small Cap Fund.
Risk/Return [Heading]	rr_RiskReturnHeading	SIMT Small Cap Fund
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Change in Performance Information for the Small Cap Fund
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	In the "Performance Information" section of the Fund Summary for the Small Cap Fund, the information regarding the Small Cap Fund's worst quarter of performance history is hereby deleted and replaced with the following:
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Worst Quarter: -23.12% (09/30/11)
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst Quarter:
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(23.12%)
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	There are no other changes in the performance information of the Small Cap Fund. PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE

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Risk/Return Bar Chart and Table.

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If the Fund has annual returns for at least one calendar year, provide a bar chart showing the Fund's annual total returns for each of the last 10 calendar years (or for the life of the Fund if less than 10 years), but only for periods subsequent to the effective date of the Fund's registration statement. Present the corresponding numerical return adjacent to each bar. If the Fund's fiscal year is other than a calendar year, include the year-to-date return information as of the end of the most recent quarter in a footnote to the bar chart. Following the bar chart, disclose the Fund's highest and lowest return for a quarter during the 10 years or other period of the bar chart.

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Risk/Return Bar Chart and Table.

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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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Risk/Return Summary Investment Objectives/Goals Include the following information, in plain English under rule 421(d) under the Securities Act, in the order and subject matter indicated

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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