Accumulated Comprehensive Income (Loss) (Schedule Of Accumulated Other Comprehensive Income (Loss), Net Of Tax) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Accumulated other comprehensive income (loss), Foreign Currency Translation Adjustments	$ 3,410	$ (674)	$ 1,818	$ 298
Accumulated other comprehensive income (loss), Unrealized Holding Gains (Losses) on Investments	2,829	2,574	1,339	960
Accumulated Other Comprehensive Income (Loss)	6,239	1,900	3,157	1,258
Total comprehensive loss, Foreign Currency Translation Adjustments	5,904	(3,195)	2,205
Total comprehensive loss, Unrealized Holding Gains (Losses) on Investments	255	1,235	379
Other Comprehensive Income (Loss), Net of Tax	6,159	(1,960)	2,584
Total comprehensive loss attributable to noncontrolling interest, Foreign Currency Translation Adjustments	(1,820)	703	(685)
Total comprehensive income attributable to noncontrolling interest, Unrealized Holding Gains (Losses) on Investments	0	0	0
Total comprehensive loss attributable to noncontrolling interest	(1,820)	703	(685)
Total comprehensive income attributable to SEI Investments Company, Foreign Currency Translation Adjustments	4,084	(2,492)	1,520
Total comprehensive (loss) attributable to SEI Investments Company, Unrealized Holding Gains (Losses) on Investments	255	1,235	379
Total comprehensive income attributable to SEI Investments Company	4,339	(1,257)	1,899
SEI Investments Company [Member]
Accumulated other comprehensive income (loss), Foreign Currency Translation Adjustments	3,861	(2,043)	1,152	(1,053)
Accumulated other comprehensive income (loss), Unrealized Holding Gains (Losses) on Investments	2,829	2,574	1,339	960
Accumulated Other Comprehensive Income (Loss)	6,690	531	2,491	(93)
Noncontrolling Interest [Member]
Accumulated other comprehensive income (loss), Foreign Currency Translation Adjustments	(451)	1,369	666	1,351
Accumulated other comprehensive income (loss), Unrealized Holding Gains (Losses) on Investments	0	0	0	0
Accumulated Other Comprehensive Income (Loss)	$ (451)	$ 1,369	$ 666	$ 1,351