Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended										9 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Sep. 30, 2013 SEI-Sponsored Open-Ended Money Market Mutual Funds [Member]	Dec. 31, 2012 SEI-Sponsored Open-Ended Money Market Mutual Funds [Member]	Sep. 30, 2013 SEI Investments (Europe) Limited [Member]	Dec. 31, 2012 SEI Investments (Europe) Limited [Member]	Sep. 30, 2013 SEI Investments Distribution Co. (SIDCO) [Member]	Dec. 31, 2012 SEI Investments Distribution Co. (SIDCO) [Member]	Sep. 30, 2013 SEI Wealth Platform [Member]	Sep. 30, 2012 SEI Wealth Platform [Member]	Mar. 28, 2013 SEI Asset Korea Co., Ltd. [Member]	Sep. 30, 2013 SEI Asset Korea Co., Ltd. [Member]	Dec. 31, 2012 SEI Asset Korea Co., Ltd. [Member]
Percentage of ownership before sale of subsidiary															56.10%
Closing date, sale of subsidiary																Mar. 28, 2013
Cash and cash equivalents	$ 527,214	$ 397,677	$ 527,214	$ 397,677	$ 452,247	$ 420,986	$ 368,819	$ 247,314									$ 48,306
Restricted cash									5,000	5,000	500	1,000
Payments to acquire software			31,044	25,894
Capitalized software weighted average remaining life	8 years 9 months		8 years 9 months										8 years 9 months
Amortization expense related to capitalized software			25,328	24,858									25,046	24,506
Net book value of SWP													307,121
Capitalized software, accumulated amortization	171,015		171,015		149,747								140,925
Capitalized software development costs in-progress associated with future releases													5,943
Software development cost useful life													15 years
Accelerated amortization for SWP abandoned components														$ 2,661
Anti-dilutive employee stock options	3,042,000	9,552,000	5,522,000	13,908,000
Antidilutive securities excluded from computation of net income, per outstanding unit, amount	$ 31.39	$ 26.51	$ 29.22	$ 23.40