Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Expense related to soft dollar arrangement	$ 3,730	$ 4,073
Cash and cash equivalents invested in SEI-sponsored money market funds	748,299		$ 744,247
Restricted cash	3,507		3,505
Capitalized software development costs	12,887	$ 16,861
Net book value of capitalized software	$ 312,272		310,405
Anti-dilutive employee stock options	6,054	11,279
Anti-dilutive employee stock options, per share	$ 52.73	$ 37.70
SEI Wealth Platform [Member]
Capitalized software development costs	$ 12,042	$ 15,178
Net book value of capitalized software	287,530
Capitalized software in progress	33,237
Amortization expense of capitalized software	$ 9,719	11,972
SEI Wealth Platform [Member] \| Weighted Average [Member]
Estimated useful life of capitalized software	8 years 4 months 24 days
Application for Investment Managers segment [Member]
Capitalized software development costs	$ 845	$ 1,683
Net book value of capitalized software	$ 24,742
Estimated useful life of capitalized software	5 years
Amortization expense of capitalized software	$ 1,301
SEI Investments (Europe) Limited [Member]
Restricted cash	3,000		3,000
SEI Investments Distribution Co. (SIDCO) [Member]
Restricted cash	507		505
SEI-Sponsored Open-Ended Money Market Mutual Funds [Member]
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 341,413		$ 401,292

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Expense Related To Soft Dollar Arrangement

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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic net Income or Loss per outstanding limited partnership unit in the future and that were not included in the computation of diluted net income per limited partnership unit, because to do so would increase net income per unit amounts or decrease loss per unit amounts for the period presented.

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Additions made to capitalized computer software costs during the period.

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Amount of expense for amortization of capitalized computer software costs.

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The carrying amount of capitalized computer software costs net of accumulated amortization as of the balance sheet date.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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