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 (Loss)	Total
Increase (Decrease) in Stockholders' Equity
Adoption of new accounting principle guidance | ASU 2016-01			$ 812		$ (812)
Adoption of new accounting principle guidance | ASU 2018-02			36		(36)
Balance at the beginning of the period at Dec. 31, 2017	$ 997	$ 301,410	1,092,394	$ (522,441)	524	$ 872,884
Balance (in shares) at Dec. 31, 2017	99,701
Increase (Decrease) in Stockholders' Equity
Net income (loss)			137,120			137,120
Recognition of equity compensation		32,871	991			33,862
Net issuance/forfeiture of nonvested shares		(22,068)		22,068
Dividends accrued			(60,568)			(60,568)
Repurchase of common stock				(89,018)		(89,018)
Other comprehensive income (loss)					(770)	(770)
Balance at the end of the period at Sep. 30, 2018	$ 997	312,213	1,170,785	(589,391)	(1,094)	893,510
Balance (in shares) at Sep. 30, 2018	99,701
Balance at the beginning of the period at Dec. 31, 2017						14,509
Increase (Decrease) in Redeemable Noncontrolling Interest
Net income (loss)						(380)
Net subscription of redeemable noncontrolling interests in sponsored funds						2,004
Balance at the end of the period at Sep. 30, 2018						16,133
Balance at the beginning of the period at Jun. 30, 2018	$ 997	302,144	1,144,090	(560,181)	(1,332)	885,718
Balance (in shares) at Jun. 30, 2018	99,701
Increase (Decrease) in Stockholders' Equity
Net income (loss)			46,305			46,305
Recognition of equity compensation		9,228	78			9,306
Net issuance/forfeiture of nonvested shares		841		(841)
Dividends accrued			(19,688)			(19,688)
Repurchase of common stock				(28,369)		(28,369)
Other comprehensive income (loss)					238	238
Balance at the end of the period at Sep. 30, 2018	$ 997	312,213	1,170,785	(589,391)	(1,094)	893,510
Balance (in shares) at Sep. 30, 2018	99,701
Balance at the beginning of the period at Jun. 30, 2018						17,052
Increase (Decrease) in Redeemable Noncontrolling Interest
Net income (loss)						208
Net redemption of redeemable noncontrolling interests in sponsored funds						(1,127)
Balance at the end of the period at Sep. 30, 2018						16,133
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 (loss)			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 (loss)						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 (loss)			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 (loss)						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