Accumulated Comprehensive Income (Loss) (Accumulated Comprehensive Income (Loss), Net Of Tax) (Details) (USD $) In Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustments	$ (1,967)		$ (1,967)		$ 1,818
Accumulated Other Comprehensive Income (Loss), Unrealized Holding Gains (Losses) on Investments	2,354		2,354		1,339
Accumulated Other Comprehensive Income (Loss)	387		387		3,157
Total Comprehensive (Loss) Income, Foreign Currency Translation Adjustments	(9,179)	4,988	(4,884)	694
Total Comprehensive Income (loss)	(8,679)	5,373	(3,869)	1,800
Total Comprehensive (Loss) Income, Foreign Currency Translation Adjustments, Attributable to SEI Investments Company			(3,785)
Total Comprehensive (Loss) Income, Unrealized Holding Gains (Losses) on Investments, Attributable to SEI Investments Company			1,015
Total Comprehensive (Loss) Income, Attributable to SEI Investments Company			(2,770)
Controlling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustments	(3,732)		(3,732)		1,152
Accumulated Other Comprehensive Income (Loss), Unrealized Holding Gains (Losses) on Investments	2,354		2,354		1,339
Accumulated Other Comprehensive Income (Loss)	(1,378)		(1,378)		2,491
Total Comprehensive (Loss) Income, Foreign Currency Translation Adjustments			(4,884)
Total Comprehensive (Loss) Income, Unrealized Holding Gains (Losses) on Investments			1,015
Total Comprehensive Income (loss)			(3,869)
Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustments	1,765		1,765		666
Accumulated Other Comprehensive Income (Loss), Unrealized Holding Gains (Losses) on Investments	0		0		0
Accumulated Other Comprehensive Income (Loss)	1,765		1,765		666
Total Comprehensive Loss, Foreign Currency Translation Adjustments, Attributable to Noncontrolling Interest			1,099
Total Comprehensive Loss, Unrealized Holding Gains (Losses) on Investments, Attributable to Noncontrolling Interest			0
Total Comprehensive Loss, Attributable to Noncontrolling Interest			$ 1,099

Accumulated appreciation or loss, net of tax, in value of the total of available-for-sale securities at the end of an accounting period.

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