Label	Element	Value
Class I Prospectus \| Simt Large Cap Value Fund
Risk/Return:	rr_RiskReturnAbstract
Supplement to Prospectus [Text Block]	rr_SupplementToProspectusTextBlock	SEI INSTITUTIONAL MANAGED TRUST Large Cap Value Fund (the "Fund") Supplement Dated July 10, 2019 to the Class I Prospectus, dated January 31, 2019, as amended on April 1, 2019, April 4, 2019 and July 1, 2019 (the "Prospectus")
Risk/Return [Heading]	rr_RiskReturnHeading	Simt Large Cap Value Fund
Bar Chart Narrative [Text Block]	rr_BarChartNarrativeTextBlock	In the Fund Summary for the Fund, under the heading titled "Performance Information," the bar chart showing changes in the Fund's performance from year to year for the past ten calendar years is hereby deleted and replaced with the following:
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	There are no other changes to the Prospectus. PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE
Class I Prospectus \| Simt Large Cap Value Fund \| CLASS I
Risk/Return:	rr_RiskReturnAbstract
Annual Return 2009	rr_AnnualReturn2009	18.23%
Annual Return 2010	rr_AnnualReturn2010	14.33%
Annual Return 2011	rr_AnnualReturn2011	(0.30%)
Annual Return 2012	rr_AnnualReturn2012	15.69%
Annual Return 2013	rr_AnnualReturn2013	34.70%
Annual Return 2014	rr_AnnualReturn2014	10.84%
Annual Return 2015	rr_AnnualReturn2015	(6.57%)
Annual Return 2016	rr_AnnualReturn2016	14.68%
Annual Return 2017	rr_AnnualReturn2017	17.13%
Annual Return 2018	rr_AnnualReturn2018	(9.92%)

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. When a Multiple Class Fund offers more than one Class in the prospectus, provide annual total returns in the bar chart for only one of those Classes. The Fund can select which Class to include (e.g., the oldest Class, the Class with the greatest net assets).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form
-Number N-1A
-Chapter A
-Section 4
-Subsection b
-Paragraph 2
-Subparagraph instructions
-Clause 3
-Exhibit a