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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 684,971	$ 623,162	$ 684,971	$ 623,162	$ 695,701	$ 679,661
Restricted cash	3,503		3,503		3,500
Capitalized software development costs			48,573	$ 33,196
Net book value of capitalized software	$ 307,116		$ 307,116		295,867
Anti-dilutive employee stock options	11,324	10,258	11,286	10,384
Anti-dilutive employee stock options, per share	$ 37.81	$ 34.11	$ 37.73	$ 34.07
SEI Wealth Platform [Member]
Capitalized software development costs			$ 40,604	$ 27,387
Net book value of capitalized software	$ 283,639		283,639
Capitalized software in progress	$ 27,344		27,344
Estimated useful life of the SEI Wealth Platform	15 years
Amortization expense related to the SEI Wealth Platform			37,324	33,387
SEI Wealth Platform [Member] | Weighted Average [Member]
Estimated useful life of the SEI Wealth Platform	4 years 8 months 12 days
Application for Investment Managers segment [Member]
Capitalized software development costs			7,969	$ 5,809
Net book value of capitalized software	$ 23,477		23,477
SEI Investments (Europe) Limited [Member]
Restricted cash	3,000		3,000		3,000
SEI Investments Distribution Co. (SIDCO) [Member]
Restricted cash	503		503		500
SEI-Sponsored Open-Ended Money Market Mutual Funds [Member]
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 298,869		$ 298,869		$ 374,760

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