Summary of Significant Accounting Policies (Net Cash Provided by Operating Activities) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accounting Policies [Abstract]
Net income	$ 132,168	$ 128,319	$ 372,689	$ 389,834
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	7,409	7,255	22,162	21,515
Amortization	12,947	12,405	38,407	36,420
Equity in earnings of unconsolidated affiliate	(37,609)	(41,726)	(112,758)	(123,406)
Distributions received from unconsolidated affiliate			123,663	138,216
Stock-based compensation	$ 5,453	$ 5,878	15,555	16,396
Provision for losses on receivables			593	(29)
Deferred income tax expense			(1,405)	8,378
Net (gain) loss from investments			(2,121)	460
Change in long-term income taxes payable			0	(9,859)
Change in other long-term liabilities			2,077	1,930
Change in other assets			(56)	(4,214)
Contract costs capitalized, net of amortization			(4,499)	(3,463)
Other			(721)	(99)
(Increase) decrease in
Receivables from investment products			(2,271)	2,263
Receivables			(34,589)	(44,878)
Other current assets			729	(5,955)
(Decrease) increase in
Accounts payable			(2,208)	3,893
Accrued liabilities			(34,087)	(9,717)
Deferred revenue			375	213
Total adjustments			8,846	28,064
Net cash provided by operating activities			$ 381,535	$ 417,898