Consolidated Statements of Stockholders' Equity and Redeemable Noncontrolling Interests - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-in Capital	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Income	Total
Balance at the beginning of the period at Dec. 31, 2018	$ 997	$ 311,264	$ 1,198,445	$ (627,587)	$ 331	$ 883,450
Balance (in shares) at Dec. 31, 2018	99,701
Increase (Decrease) in Stockholders' Equity
Net income			99,056			99,056
Recognition of equity compensation		25,573	297			25,870
Net issuance/forfeiture of nonvested shares		(33,699)		33,699
Dividends accrued			(55,121)			(55,121)
Repurchase of common stock				(116,677)		(116,677)
Other comprehensive income (loss)					2,952	2,952
Balance at the end of the period at Sep. 30, 2019	$ 997	303,138	1,242,677	(710,565)	3,283	839,530
Balance (in shares) at Sep. 30, 2019	99,701
Balance at the beginning of the period at Dec. 31, 2018						11,463
Increase (Decrease) in Redeemable Noncontrolling Interest
Net income						1,763
Net subscription of redeemable noncontrolling interests in sponsored funds						3,687
Balance at the end of the period at Sep. 30, 2019						16,913
Balance at the beginning of the period at Jun. 30, 2019	$ 997	294,487	1,227,314	(669,223)	3,081	856,656
Balance (in shares) at Jun. 30, 2019	99,701
Increase (Decrease) in Stockholders' Equity
Net income			33,054			33,054
Recognition of equity compensation		8,024	57			8,081
Net issuance/forfeiture of nonvested shares		627		(627)
Dividends accrued			(17,748)			(17,748)
Repurchase of common stock				(40,715)		(40,715)
Other comprehensive income (loss)					202	202
Balance at the end of the period at Sep. 30, 2019	$ 997	303,138	1,242,677	(710,565)	3,283	839,530
Balance (in shares) at Sep. 30, 2019	99,701
Balance at the beginning of the period at Jun. 30, 2019						15,115
Increase (Decrease) in Redeemable Noncontrolling Interest
Net income						445
Net subscription of redeemable noncontrolling interests in sponsored funds						1,353
Balance at the end of the period at Sep. 30, 2019						16,913
Balance at the beginning of the period at Dec. 31, 2019	$ 997	312,693	1,241,598	(749,625)	3,234	808,897
Balance (in shares) at Dec. 31, 2019	99,701
Increase (Decrease) in Stockholders' Equity
Net income			77,333			77,333
Recognition of equity compensation		20,895	110			21,005
Net issuance/forfeiture of nonvested shares		(37,368)		37,368
Dividends accrued			(48,363)			(48,363)
Repurchase of common stock				(112,052)		(112,052)
Other comprehensive income (loss)					691	691
Balance at the end of the period at Sep. 30, 2020	$ 997	296,220	1,270,678	(824,309)	3,925	747,511
Balance (in shares) at Sep. 30, 2020	99,701
Balance at the beginning of the period at Dec. 31, 2019						19,205
Increase (Decrease) in Redeemable Noncontrolling Interest
Net income						2,201
Net deconsolidation of redeemable noncontrolling interests in sponsored funds						(21,406)
Balance at the beginning of the period at Jun. 30, 2020	$ 997	289,439	1,255,770	(783,990)	4,487	766,703
Balance (in shares) at Jun. 30, 2020	99,701
Increase (Decrease) in Stockholders' Equity
Net income			30,523			30,523
Recognition of equity compensation		6,514	20			6,534
Net issuance/forfeiture of nonvested shares		267		(267)
Dividends accrued			(15,635)			(15,635)
Repurchase of common stock				(40,052)		(40,052)
Other comprehensive income (loss)					(562)	(562)
Balance at the end of the period at Sep. 30, 2020	$ 997	$ 296,220	$ 1,270,678	$ (824,309)	$ 3,925	747,511
Balance (in shares) at Sep. 30, 2020	99,701
Balance at the beginning of the period at Jun. 30, 2020						25,857
Increase (Decrease) in Redeemable Noncontrolling Interest
Net income						723
Net deconsolidation of redeemable noncontrolling interests in sponsored funds						$ (26,580)