Label	Element	Value
Class A Prospectus \| SIMT Large Cap Fund
Risk/Return:	rr_RiskReturnAbstract
Supplement [Text Block]	ck0000804239_SupplementTextBlock	SEI INSTITUTIONAL MANAGED TRUST Large Cap Fund (each, a "Fund" and together, the "Funds") Supplement Dated July 17, 2015 to the Class A Shares Prospectus (the "Prospectus") dated January 31, 2015 This Supplement provides new and additional information beyond that contained in the Class A 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 Large 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.
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Changes to the Large Cap Fund's Principal Investment Strategies and Sub-Advisers In the Large Cap Fund's "Principal Investment Strategies" section, the text is hereby deleted and replaced with the following: Under normal circumstances, the Large Cap Fund will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in equity securities of large companies. For purposes of this Fund, a large company is a company with a market capitalization in the range of companies in the Russell 1000 Index (between $275 million and $648 billion as of December 31, 2014) at the time of purchase. The market capitalization range and the composition of the Russell 1000 Index are subject to change. These securities may include common stocks, preferred stocks, warrants, American depositary receipts (ADRs) and exchange-traded funds (ETFs). The Fund may also, to a lesser extent, invest in common and preferred stocks of small capitalization companies. The Fund may invest up to 20% of its assets in foreign securities. The Fund uses a multi-manager approach, relying primarily on a number of sub-advisers (each, a Sub-Adviser and collectively, the Sub-Advisers) with differing investment philosophies and strategies to manage portions of the Fund's portfolio under the general supervision of SEI Investments Management Corporation, the Fund's adviser (SIMC or the Adviser). Assets of the Fund not allocated to Sub-Advisers are managed directly by SIMC. In managing its portion of the Fund's assets, SIMC or each Sub-Adviser may apply a variety of quantitative and/or fundamental investment styles. A quantitative investment style generally involves a systematic or rules-based approach to selecting investments based on specific measurable factors. A fundamental investment style generally involves selecting investments through research and analysis of financial statements, relevant industry and economic data, or other characteristics. The Fund implements the investment recommendations of SIMC and its Sub-Advisers through the use of an overlay manager appointed by SIMC. Each Sub-Adviser provides a model portfolio to the Fund on an ongoing basis that represents that Sub-Adviser's recommendation as to the securities to be purchased, sold or retained by the Fund. The overlay manager then constructs a portfolio for the Fund that represents the aggregation of the model portfolios of the Sub-Advisers and SIMC with the weighting of each Sub-Adviser's model in the total portfolio determined by SIMC. Pursuant to direction from SIMC, the overlay manager has limited authority to vary from the models. For example, SIMC may direct the overlay manager to adjust the portfolio to implement SIMC's forward looking views regarding various portfolio characteristics or factors, or for risk management purposes. The overlay manager may also vary the portfolio implementation to seek trading cost efficiencies, loss harvesting, portfolio rebalancing or other portfolio construction objectives as directed by SIMC. There are no other changes to the principal investment strategies or portfolio management of the Large Cap Fund. 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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