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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 638,253	$ 643,260	$ 679,661	$ 667,446
Restricted cash	5,500		5,500
Capitalized software development costs	9,477	$ 7,989
Net book value of SEI Wealth Platform	$ 289,044		290,522
Anti-dilutive employee stock options	10,506	10,051
Antidilutive securities excluded from computation of net income, per outstanding unit, amount	$ 34.05	$ 30.02
SEI Investments Distribution Co. (SIDCO) [Member]
Restricted cash	$ 500		500
SEI Investments (Europe) Limited [Member]
Restricted cash	5,000		5,000
SEI-Sponsored Open-Ended Money Market Mutual Funds [Member]
Cash and cash equivalents invested in SEI-sponsored money market funds	402,910		$ 448,957
SEI Wealth Platform [Member]
Capitalized software development costs	7,645	$ 7,330
Net book value of SEI Wealth Platform	$ 282,311
Estimated useful life of the SEI Wealth Platform	15 years
Amortization expense related to the SEI Wealth Platform	$ 10,955	10,301
Technological capabilities for Investment Managers segment [Member]
Capitalized software development costs	$ 1,832	$ 659
Weighted Average [Member] | SEI Wealth Platform [Member]
Estimated useful life of the SEI Wealth Platform	6 years 3 months

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