Summary of Significant Accounting Policies (Net Cash Provided by Operating Activities) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accounting Policies [Abstract]
Net income	$ 111,096	$ 132,168	$ 321,404	$ 372,689
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	7,945	7,409	23,058	22,162
Amortization	13,200	12,947	39,417	38,407
Equity in earnings of unconsolidated affiliate	(28,305)	(37,609)	(86,488)	(112,758)
Distributions received from unconsolidated affiliate			119,821	123,663
Stock-based compensation	6,467	5,453	20,458	15,555
Provision for losses on receivables			109	593
Deferred income tax expense			(7,731)	(1,405)
Net loss (gain) from investments	$ (776)	$ (611)	1,310	(2,121)
Change in other long-term liabilities			(4,442)	2,077
Change in other assets			(965)	(56)
Contract costs capitalized, net of amortization			(2,842)	(4,499)
Other			(1,053)	(721)
(Increase) decrease in
Receivables from investment products			2,521	(2,271)
Receivables			(37,921)	(34,589)
Other current assets			(6,186)	729
(Decrease) increase in
Accounts payable			8,615	(2,208)
Accrued liabilities			8,809	(34,087)
Deferred revenue			(1,370)	375
Total adjustments			75,120	8,846
Net cash provided by operating activities			$ 396,524	$ 381,535