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)			46,337			46,337
Recognition of equity compensation		12,065	209			12,274
Net issuance/forfeiture of nonvested shares		(29,707)		29,707
Dividends accrued, $0.25 per share			(20,866)			(20,866)
Repurchase of common stock				(20,507)		(20,507)
Other comprehensive income (loss)					(1,154)	(1,154)
Balance at the end of the period at Mar. 31, 2018	$ 997	283,768	1,118,922	(513,241)	(1,478)	888,968
Balance (in shares) at Mar. 31, 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)						(366)
Net subscription of redeemable noncontrolling interests in sponsored funds						4,427
Balance at the end of the period at Mar. 31, 2018						18,570
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)			32,053			32,053
Recognition of equity compensation		9,608	93			9,701
Net issuance/forfeiture of nonvested shares		(30,000)		30,000
Dividends accrued, $0.25 per share			(19,025)			(19,025)
Repurchase of common stock				(39,139)		(39,139)
Other comprehensive income (loss)					1,564	1,564
Balance at the end 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
Balance at the beginning of the period at Dec. 31, 2018						11,463
Increase (Decrease) in Redeemable Noncontrolling Interest
Net income (loss)						705
Net subscription of redeemable noncontrolling interests in sponsored funds						768
Balance at the end of the period at Mar. 31, 2019						$ 12,936