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	9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Accounting Policies [Abstract]
Net income	$ 83,983	$ 67,195	$ 241,616	$ 222,959
Depreciation	5,579	5,616	16,494	17,050
Amortization	9,813	8,829	28,611	25,498
Equity in earnings of unconsolidated affiliates	(37,963)	(31,028)	(104,080)	(86,204)
Distributions received from unconsolidated affiliate			103,592	109,173
Stock-based compensation	3,786	11,097	9,481	26,997
Provision for losses on receivables			387	(180)
Deferred income tax expense			4,768	(21,443)
Gain on sale of SEI AK	0	0	(5,582)	(22,112)
Net gain from investments			(49)	(638)
Change in other long-term liabilities			2,663	1,671
Change in other assets			(2,769)	80
Other			(4,665)	(4,199)
Decrease (increase) in Restricted cash for broker-dealer operations			0	500
Decrease (increase) in Receivables from regulated investment companies			(7,169)	(8,495)
Decrease (increase) in Receivables			(24,951)	(17,255)
Decrease (increase) in Other current assets			(4,862)	(1,208)
Increase (decrease) in Accounts payable			(9,723)	(3,960)
Increase (decrease) in Accrued liabilities			(1,988)	3,149
Increase (decrease) in Deferred revenue			(879)	(1,088)
Total adjustments			(721)	17,336
Net cash provided by operating activities			$ 240,895	$ 240,295

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The sum of adjustments which are added to or deducted from net income or loss, including the portion attributable to noncontrolling interest, to reflect cash provided by or used in operating activities, in accordance with the indirect cash flow method.