Summary of Significant Accounting Policies (Adjustments To Reconcile Net Income To Net Cash Provided By Operating Activities) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accounting Policies [Abstract]
Net income	$ 86,240	$ 82,813	$ 170,851	$ 157,633
Depreciation	5,843	5,331	11,838	10,915
Amortization	10,611	9,584	20,969	18,798
Equity in earnings of unconsolidated affiliates	(37,289)	(34,226)	(71,322)	(66,117)
Distributions received from unconsolidated affiliate			74,847	69,470
Stock-based compensation	3,859	2,872	7,609	5,695
Provision for losses on receivables			(128)	227
Deferred income tax expense			482	5,270
Gain on sale of SEI AK	$ 0	$ 0	(2,791)	(5,582)
Net gain from investments			(212)	(527)
Change in other long-term liabilities			914	1,113
Change in other assets			(643)	(2,564)
Other			(1,269)	238
Decrease (increase) in Receivables from regulated investment companies			1,473	(4,680)
Decrease (increase) in Receivables			(21,502)	(21,245)
Decrease (increase) in Other current assets			(6,745)	(5,392)
Increase (decrease) in Accounts payable			(3,813)	(7,765)
Increase (decrease) in Accrued liabilities			(27,412)	(13,061)
Increase (decrease) in Deferred revenue			3,638	(639)
Total adjustments			(14,067)	(15,846)
Net cash provided by operating activities			$ 156,784	$ 141,787