Quarterly Financial Data (SIV-Related Gains (Losses)) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Revenues	$ 261,635	$ 251,752	$ 241,237	$ 237,898	$ 226,247	$ 233,227	$ 237,760	$ 232,493	$ 992,522	$ 929,727	$ 900,835
Income before income taxes	87,544	83,501	78,501	79,950	68,603	74,900	82,820	92,164	329,496	318,487	369,781
Net income attributable to SEI	56,585	50,743	49,555	49,965	44,396	49,232	53,603	57,728	206,848	204,959	231,687
Basic earnings per common share	$ 0.33	$ 0.29	$ 0.28	$ 0.28	$ 0.25	$ 0.27	$ 0.29	$ 0.31	$ 1.19	$ 1.12	$ 1.23
Diluted earnings per common share	$ 0.32	$ 0.29	$ 0.28	$ 0.28	$ 0.25	$ 0.27	$ 0.29	$ 0.31	$ 1.18	$ 1.11	$ 1.22
Effective tax rates	35.00%	39.00%	36.60%	37.20%	34.70%	33.80%	34.70%	37.10%	36.90%	35.20%	37.00%
SIV Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Diluted earnings per common share	$ 0.02	$ 0.01	$ 0.00	$ 0.01	$ 0.00	$ 0.00	$ 0.01	$ 0.02
Total SIV-related gains (losses)	$ 5,897	$ 3,389	$ 1,072	$ 2,882	$ (738)	$ (792)	$ (1,944)	$ 6,864