Label	Element	Value
Class Y Prospectus \| SIMT Small Cap Fund
Risk/Return:	rr_RiskReturnAbstract
Supplement [Text Block]	ck0000804239_SupplementTextBlock	SEI INSTITUTIONAL MANAGED TRUST Small Cap Fund (each, a "Fund" and together, the "Funds") Supplement Dated July 17, 2015 to the Class Y Shares Prospectus (the "Prospectus") dated January 31, 2015 This Supplement provides new and additional information beyond that contained in the Class Y Shares 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 Small Cap Fund
Expense [Heading]	rr_ExpenseHeading	Removal of Redemption Fees
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	In the Fund summary for each Fund, under the sub-heading entitled "Shareholder Fees," the table referencing Redemption Fees is hereby deleted. There are no other changes in the Fees and Expenses of the Funds.
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE

- Definition

Risk/Return Summary Fee Table Includes the following information, in plain English under rule 421(d) under the Securities Act, after Item 2 Fees and expenses of the Fund This table describes the fees and expenses that you may pay if you buy and hold shared of the Fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $[_____] in [name of fund family] funds. Shareholder Fees (fees paid directly from your investment) Example 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 you redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return per year and that the Fund's operating expenses remained the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be You would pay the following expenses if you did not redeem your shares The Example does not reflect sales charges (loads) on reinvested dividends [and other distributions]. If these sales charges (loads) were included, your costs would be higher. Portfolio Turnover The Fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover may indicate higher transaction costs. 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 __% of the average value of its whole portfolio. Instructions. A.3.instructions.6 New Funds. For purposes of this Item, a "New Fund" is a Fund that does not include in Form N-1A financial statements reporting operating results or that includes financial statements for the Fund's initial fiscal year reporting operating results for a period of 6 months or less. The following Instructions apply to New Funds.

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