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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, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 661,372	$ 638,253	$ 695,701	$ 679,661
Restricted cash	3,500		3,500
Capitalized software development costs	16,861	$ 9,477
Net book value of SEI Wealth Platform	$ 300,756		295,867
Anti-dilutive employee stock options	11,279	10,506
Antidilutive securities excluded from computation of net income, per outstanding unit, amount	$ 37.70	$ 34.05
SEI Investments Distribution Co. (SIDCO) [Member]
Restricted cash	$ 500		500
SEI Investments (Europe) Limited [Member]
Restricted cash	3,000		3,000
SEI-Sponsored Open-Ended Money Market Mutual Funds [Member]
Cash and cash equivalents invested in SEI-sponsored money market funds	313,762		$ 374,760
SEI Wealth Platform [Member]
Capitalized software development costs	15,178	$ 7,645
Net book value of SEI Wealth Platform	$ 283,565
Estimated useful life of the SEI Wealth Platform	15 years
Amortization expense related to the SEI Wealth Platform	$ 11,972	10,955
New application for Investment Managers segment [Member]
Capitalized software development costs	$ 1,683	$ 1,832
Weighted Average [Member] | SEI Wealth Platform [Member]
Estimated useful life of the SEI Wealth Platform	5 years 2 months 27 days

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