Summary of Significant Accounting Policies (Net Cash Provided by Operating Activities) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accounting Policies [Abstract]
Net income	$ 109,242	$ 113,981
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	7,473	7,331
Amortization	13,077	12,679
Equity in earnings of unconsolidated affiliate	(29,907)	(37,317)
Distributions received from unconsolidated affiliate	52,036	33,237
Stock-based compensation	6,929	5,038
Provision for losses on receivables	80	(85)
Deferred income tax expense	(1,879)	(984)
Net loss (gain) from investments	(3,989)	1,279
Change in other long-term liabilities	360	277
Change in other assets	(4,658)	1,334
Contract costs capitalized, net of amortization	(1,769)	(138)
Other	(1,612)	(63)
(Increase) decrease in
Receivables from investment products	2,553	(2,693)
Receivables	(17,852)	(18,304)
Other current assets	(9,728)	574
(Decrease) increase in
Accounts payable	8,663	(5,678)
Accrued liabilities	(36,818)	(47,836)
Deferred revenue	(1,207)	(175)
Total adjustments	(10,270)	(54,082)
Net cash provided by operating activities	$ 98,972	$ 59,899