Summary of Significant Accounting Policies (Net Cash Provided by Operating Activities) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accounting Policies [Abstract]
Net income	$ 101,066	$ 126,540	$ 210,308	$ 240,521
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	7,640	7,422	15,113	14,753
Amortization	13,140	12,781	26,217	25,460
Equity in earnings of unconsolidated affiliate	(28,276)	(37,832)	(58,183)	(75,149)
Distributions received from unconsolidated affiliate			84,502	77,288
Stock-based compensation	7,062	5,064	13,991	10,102
Provision for losses on receivables			(122)	383
Deferred income tax expense			(5,459)	(2,050)
Net loss (gain) from investments	$ (1,903)	$ (231)	2,086	(1,510)
Change in other long-term liabilities			(1,448)	1,129
Change in other assets			(6,135)	249
Contract costs capitalized, net of amortization			(2,117)	(1,125)
Other			(1,217)	122
(Increase) decrease in
Receivables from investment products			4,292	(2,538)
Receivables			(11,134)	(20,192)
Other current assets			(2,716)	3,708
(Decrease) increase in
Accounts payable			6,357	(5,426)
Accrued liabilities			(9,367)	(47,631)
Deferred revenue			(836)	(449)
Total adjustments			53,824	(22,876)
Net cash provided by operating activities			$ 264,132	$ 217,645