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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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Cash and cash equivalents invested in SEI-sponsored money market funds	$ 647,589	$ 577,914	$ 647,589	$ 577,914	$ 667,446	$ 578,273
Restricted cash	6,319		6,319		5,801
Capitalized software development costs			22,508	$ 26,193
Net book value of SEI Wealth Platform	$ 294,380		$ 294,380		309,040
Anti-dilutive employee stock options	9,938	4,411	9,999	4,416
Antidilutive securities excluded from computation of net income, per outstanding unit, amount	$ 30.02	$ 32.38	$ 30.02	$ 32.37
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	401,634		401,634		$ 435,268
SEI Wealth Platform [Member]
Capitalized software development costs			19,344	$ 26,193
Net book value of SEI Wealth Platform	$ 291,216		291,216
Estimated useful life of the SEI Wealth Platform	15 years
Amortization expense related to the SEI Wealth Platform			$ 31,635	$ 28,345
Income statement classification for capitalized software write off related to the SEI Wealth Platform	Facilities, supplies and other costs
Weighted Average [Member] | SEI Wealth Platform [Member]
Estimated useful life of the SEI Wealth Platform	6 years 9 months
Capitalized software [Member] | SEI Wealth Platform [Member]
Loss on write off of capitalized software related to the SEI Wealth Platform	$ 5,533
Private Banks [Member] | SEI Wealth Platform [Member]
Segment classification for capitalized software write off	Private Banks
Investment Advisors [Member] | SEI Wealth Platform [Member]
Segment classification for capitalized software write off	Investment Advisors

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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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