Summary of Significant Accounting Policies (Calculation Of Basic And Diluted Earnings Per Share) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Basic earnings per common share, Net income attributable to SEI (Numerator)	$ 77,097	$ 83,983	$ 82,813	$ 74,820	$ 65,515	$ 67,195	$ 83,494	$ 71,920	$ 318,713	$ 288,124	$ 206,848
Shares used to compute basic earnings per share									168,246	171,561	174,295
Basic earnings per common share	$ 0.46	$ 0.50	$ 0.49	$ 0.44	$ 0.39	$ 0.39	$ 0.48	$ 0.42	$ 1.89	$ 1.68	$ 1.19
Dilutive effect of stock options, Net income attributable to SEI (Numerator)									0	0	0
Dilutive effect of stock options, shares									4,319	4,157	1,577
Diluted earnings per common share, Net income attributable to SEI (Numerator)									$ 318,713	$ 288,124	$ 206,848
Shares used to compute diluted earnings per share									172,565	175,718	175,872
Diluted earnings per common share	$ 0.45	$ 0.49	$ 0.48	$ 0.43	$ 0.37	$ 0.38	$ 0.47	$ 0.41	$ 1.85	$ 1.64	$ 1.18