Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended				3 Months Ended	12 Months Ended												12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012 Global Wealth Platform [Member]	Dec. 31, 2012 Global Wealth Platform [Member]	Dec. 31, 2012 Minimum [Member]	Dec. 31, 2012 Maximum [Member]	Dec. 31, 2012 Noncontrolling Interest [Member]	Dec. 31, 2012 Equity Method Investee [Member] Minimum [Member]	Dec. 31, 2012 Equity Method Investee [Member] Maximum [Member]	Dec. 31, 2012 SEI-Sponsored Open-Ended Money Market Mutual Funds [Member]	Dec. 31, 2011 SEI-Sponsored Open-Ended Money Market Mutual Funds [Member]	Dec. 31, 2012 SEI Investments (Europe) Limited [Member]	Dec. 31, 2011 SEI Investments (Europe) Limited [Member]	Dec. 31, 2012 SEI Investments Distribution Co. (SIDCO) [Member]	Dec. 31, 2011 SEI Investments Distribution Co. (SIDCO) [Member]	Dec. 31, 2012 Building and Building Improvements [Member] Minimum [Member]	Dec. 31, 2012 Building and Building Improvements [Member] Maximum [Member]	Dec. 31, 2012 Computer Equipment [Member] Minimum [Member]	Dec. 31, 2012 Computer Equipment [Member] Maximum [Member]	Dec. 31, 2012 Furniture and Fixtures [Member] Minimum [Member]	Dec. 31, 2012 Furniture and Fixtures [Member] Maximum [Member]
Noncontrolling Interest, Ownership Percentage by Parent									100.00%
Equity Method Investment, Ownership Percentage								50.00%		20.00%	50.00%
Cash and cash equivalents	$ 452,247	$ 420,986	$ 496,292	$ 590,877								$ 247,314	$ 281,760
Restricted cash														5,000	5,000	1,000	1,000
Property, Plant and Equipment, Useful Life																		25 years	39 years	3 years	5 years	3 years	5 years
Net book value of GWP					304,271	304,271
Capitalized software, accumulated amortization	149,747	117,100			119,939	119,939
Capitalized software development costs in-progress associated with future releases					2,795	2,795
Payments to Acquire Software	31,004	40,954	38,729
Software Development Cost Useful Life						15 years	3 years	15 years
Capitalized Software Weighted Average Remaining Life	9 years 6 months				9 years 6 months	9 years 6 months
Amortization expense related to capitalized software	32,647	26,153	23,053
Accelerated amortization for GWP abandoned components					2,661
Capitalized Computer Software, Impairments	0	0
Foreign Currency Transaction Gain (Loss), Realized	$ 0	$ 0	$ 0
Anti-dilutive employee stock options	13,202,000	15,914,000	13,181,000
Antidilutive Securities Excluded from Computation of Net Income, Per Outstanding Unit, Amount	$ 24.76	$ 23.07	$ 23.79