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			66,002			66,002
Recognition of equity compensation		17,549	240			17,789
Net issuance/forfeiture of nonvested shares		(34,326)		34,326
Dividends accrued			(37,373)			(37,373)
Repurchase of common stock				(75,962)		(75,962)
Other comprehensive income					2,750	2,750
Balance at the end 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
Balance at the beginning of the period at Dec. 31, 2018						11,463
Increase (Decrease) in Redeemable Noncontrolling Interest
Net income						1,318
Net subscription of redeemable noncontrolling interests in sponsored funds						2,334
Balance at the end of the period at Jun. 30, 2019						15,115
Balance at the beginning of the period at Mar. 31, 2019	$ 997	290,872	1,211,566	(636,726)	1,895	868,604
Balance (in shares) at Mar. 31, 2019	99,701
Increase (Decrease) in Stockholders' Equity
Net income			33,948			33,948
Recognition of equity compensation		7,941	148			8,089
Net issuance/forfeiture of nonvested shares		(4,326)		4,326
Dividends accrued			(18,348)			(18,348)
Repurchase of common stock				(36,823)		(36,823)
Other comprehensive income					1,186	1,186
Balance at the end 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
Balance at the beginning of the period at Mar. 31, 2019						12,936
Increase (Decrease) in Redeemable Noncontrolling Interest
Net income						614
Net subscription of redeemable noncontrolling interests in sponsored funds						1,565
Balance at the end of the period at Jun. 30, 2019						15,115
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			46,810			46,810
Recognition of equity compensation		14,381	90			14,471
Net issuance/forfeiture of nonvested shares		(37,635)		37,635
Dividends accrued			(32,728)			(32,728)
Repurchase of common stock				(72,000)		(72,000)
Other comprehensive income					1,253	1,253
Balance at the end 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
Balance at the beginning of the period at Dec. 31, 2019						19,205
Increase (Decrease) in Redeemable Noncontrolling Interest
Net income						1,478
Net subscription of redeemable noncontrolling interests in sponsored funds						5,174
Balance at the end of the period at Jun. 30, 2020						25,857
Balance at the beginning of the period at Mar. 31, 2020	$ 997	282,401	1,247,084	(765,579)	755	765,658
Balance (in shares) at Mar. 31, 2020	99,701
Increase (Decrease) in Stockholders' Equity
Net income			24,824			24,824
Recognition of equity compensation		6,688	19			6,707
Net issuance/forfeiture of nonvested shares		350		(350)
Dividends accrued			(16,157)			(16,157)
Repurchase of common stock				(18,061)		(18,061)
Other comprehensive income					3,732	3,732
Balance at the end 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
Balance at the beginning of the period at Mar. 31, 2020						19,070
Increase (Decrease) in Redeemable Noncontrolling Interest
Net income						3,000
Net subscription of redeemable noncontrolling interests in sponsored funds						3,787
Balance at the end of the period at Jun. 30, 2020						$ 25,857