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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 594,767	$ 624,841	$ 594,767	$ 624,841	$ 679,661	$ 667,446
Restricted cash	5,500		5,500		5,500
Capitalized software development costs			19,597	$ 16,069
Net book value of SEI Wealth Platform	$ 288,070		$ 288,070		290,522
Anti-dilutive employee stock options	10,388	10,007	10,447	10,029
Antidilutive securities excluded from computation of net income, per outstanding unit, amount	$ 34.06	$ 30.02	$ 34.05	$ 30.02
SEI Investments Distribution Co. (SIDCO) [Member]
Restricted cash	$ 500		$ 500		500
SEI Investments (Europe) Limited [Member]
Restricted cash	5,000		5,000		5,000
SEI-Sponsored Open-Ended Money Market Mutual Funds [Member]
Cash and cash equivalents invested in SEI-sponsored money market funds	311,152		311,152		$ 448,957
SEI Wealth Platform [Member]
Capitalized software development costs			16,120	$ 13,649
Net book value of SEI Wealth Platform	$ 279,692		279,692
Estimated useful life of the SEI Wealth Platform	15 years
Amortization expense related to the SEI Wealth Platform			22,049	20,855
New application for Investment Managers segment [Member]
Capitalized software development costs			$ 3,477	$ 2,420
Weighted Average [Member] | SEI Wealth Platform [Member]
Estimated useful life of the SEI Wealth Platform	6 years

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