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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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 660,362	$ 594,767	$ 660,362	$ 594,767	$ 695,701	$ 679,661
Restricted cash	3,502		3,502		3,500
Capitalized software development costs			33,292	$ 19,597
Net book value of capitalized software	$ 304,673		$ 304,673		295,867
Anti-dilutive employee stock options	11,255	10,388	11,267	10,447
Anti-dilutive employee stock options, per share	$ 37.68	$ 34.06	$ 37.69	$ 34.05
SEI Wealth Platform [Member]
Capitalized software development costs			$ 27,994	$ 16,120
Net book value of capitalized software	$ 283,867		283,867
Estimated useful life of the SEI Wealth Platform	15 years
Amortization expense related to the SEI Wealth Platform			24,486	22,049
SEI Wealth Platform [Member] \| Weighted Average [Member]
Estimated useful life of the SEI Wealth Platform	5 years
Application for Investment Managers segment [Member]
Capitalized software development costs			5,298	$ 3,477
Net book value of capitalized software	$ 20,806		20,806
SEI Investments (Europe) Limited [Member]
Restricted cash	3,000		3,000		3,000
SEI Investments Distribution Co. (SIDCO) [Member]
Restricted cash	502		502		500
SEI-Sponsored Open-Ended Money Market Mutual Funds [Member]
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 230,721		$ 230,721		$ 374,760