Consolidated Statements Of Changes In Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock [Member]	Capital In Excess Of Par Value [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	SEI Investments Company [Member]	Noncontrolling Interest [Member]
Beginning balance at Dec. 31, 2009		$ 1,902	$ 522,080	$ 384,483	$ 1,258
Beginning balance, shares at Dec. 31, 2009		190,208,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to SEI	231,687			231,687
Purchase and retirement of common stock		(58)	(13,426)	(107,498)
Purchase and retirement of common stock, shares	5,814,000	(5,814,000)
Issuance of common stock under the employee stock purchase plan		1	1,902
Issuance of common stock under the employee stock purchase plan, shares		114,000
Issuance of common stock upon exercise of stock options		16	26,177
Issuance of common stock upon exercise of stock options, shares	(1,633,000)	1,633,000
Stock-based compensation	26,783		26,783
Tax benefit on stock options exercised			1,877
Dividends declared	37,513			(37,513)
Other comprehensive (loss) income	2,584				1,899
Total SEI Investments Shareholders' Equity						1,041,570
Noncontrolling Interest	15,155						15,155
Total Equity at Dec. 31, 2010	1,056,725	1,861	565,393	471,159	3,157
Ending balance, shares at Dec. 31, 2010		186,141,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to SEI	204,959			204,959
Purchase and retirement of common stock		(111)	(27,196)	(183,858)
Purchase and retirement of common stock, shares	11,109,000	(11,109,000)
Issuance of common stock under the employee stock purchase plan		1	1,892
Issuance of common stock under the employee stock purchase plan, shares		110,000
Issuance of common stock upon exercise of stock options		14	22,314
Issuance of common stock upon exercise of stock options, shares	(1,364,000)	1,364,000
Stock-based compensation	14,112		14,112
Tax benefit on stock options exercised			1,434
Dividends declared	48,558			(48,558)
Other comprehensive (loss) income	(1,960)				(1,257)
Total SEI Investments Shareholders' Equity	1,025,316					1,025,316
Noncontrolling Interest	16,143						16,143
Total Equity at Dec. 31, 2011	1,041,459	1,765	577,949	443,702	1,900
Ending balance, shares at Dec. 31, 2011	176,506,000	176,506,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to SEI	206,848			206,848
Purchase and retirement of common stock		(75)	(19,370)	(135,819)
Purchase and retirement of common stock, shares	7,528,000	(7,528,000)
Issuance of common stock under the employee stock purchase plan		1	1,794
Issuance of common stock under the employee stock purchase plan, shares	11,580,000	105,000
Issuance of common stock upon exercise of stock options		31	47,613
Issuance of common stock upon exercise of stock options, shares	(3,136,000)	3,137,000
Stock-based compensation	15,736		15,736
Tax benefit on stock options exercised			583
Dividends declared	108,817			(108,817)
Other comprehensive (loss) income	6,159				4,339
Total SEI Investments Shareholders' Equity	1,038,180					1,038,180
Noncontrolling Interest	19,149						19,149
Total Equity at Dec. 31, 2012	$ 1,057,329	$ 1,722	$ 624,305	$ 405,914	$ 6,239
Ending balance, shares at Dec. 31, 2012	172,220,000	172,220,000