Subsequent Events (Narrative) (Details) (USD $) In Thousands, unless otherwise specified					1 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010	Jul. 31, 2012 SEI Asset Korea [Member]	Jun. 30, 2012 SEI Asset Korea [Member]
Subsequent Event [Line Items]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Changes, Sale of Interest by Parent					56.10%
Assets	$ 1,264,862	$ 1,294,559				$ 48,776
Cash and Cash Equivalents, at Carrying Value	$ 347,494	$ 420,986	$ 442,311	$ 496,292		$ 42,553