Summary of Significant Accounting Policies (Adjustments To Reconcile Net Income To Net Cash Provided By Operating Activities) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Accounting Policies [Abstract]
Net income	$ 82,813	$ 83,494	$ 157,633	$ 155,764
Depreciation	5,331	5,730	10,915	11,434
Amortization	9,584	8,427	18,798	16,669
Equity in earnings of unconsolidated affiliates	(34,226)	(27,588)	(66,117)	(55,176)
Distributions received from unconsolidated affiliate			69,470	53,797
Stock-based compensation	2,872	10,607	5,695	15,900
Provision for losses on receivables			227	483
Deferred income tax expense			5,270	(10,000)
Gain on sale of SEI AK	0	0	(5,582)	(22,112)
Net gain from investments			(527)	(103)
Change in other long-term liabilities			1,113	825
Change in other assets			(2,564)	559
Other			1,443	(7,251)
Decrease (increase) in Restricted cash for broker-dealer operations			0	500
Decrease (increase) in Receivables from regulated investment companies			(4,680)	(4,874)
Decrease (increase) in Receivables			(21,245)	(16,407)
Decrease (increase) in Other current assets			(5,392)	344
Increase (decrease) in Accounts payable			(7,765)	(9,433)
Increase (decrease) in Accrued liabilities			(13,061)	(6,467)
Increase (decrease) in Deferred revenue			(639)	(1,138)
Total adjustments			(14,641)	(32,450)
Net cash provided by operating activities			$ 142,992	$ 123,314