Consolidated Statement of Stockholders’ Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-in Capital	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Income (Loss)	Total
Balance at the beginning of the period at Dec. 31, 2015	$ 997	$ 331,611	$ 1,085,248	$ (566,256)	$ (5,145)	$ 846,455
Balance (in shares) at Dec. 31, 2015	99,701
Increase (Decrease) in Stockholders' Equity
Net income (loss)			156,695			156,695
Recognition of equity compensation		51,382	132			51,514
Net issuance/forfeiture of nonvested shares		(84,741)		84,741
Dividends accrued, $1.84, $1.63 and $1.00 per share for 2016, 2017 and 2018, respectively			(152,953)			(152,953)
Tax impact of share-based payment arrangements		(6,344)				(6,344)
Repurchase of common stock				(49,753)		(49,753)
Other comprehensive income (loss)					(1,612)	(1,612)
Balance at the end of the period at Dec. 31, 2016	$ 997	291,908	1,089,122	(531,268)	(6,757)	844,002
Balance (in shares) at Dec. 31, 2016	99,701
Increase (Decrease) in Redeemable Noncontrolling Interest
Adoption of consolidation guidance on January 1, 2016 - redeemable noncontrolling interests in sponsored funds						14,330
Net income (loss)						1,414
Net redemption of redeemable noncontrolling interests in sponsored funds						(5,091)
Balance at the end of the period at Dec. 31, 2016						10,653
Increase (Decrease) in Stockholders' Equity
Net income (loss)			141,279			141,279
Recognition of equity compensation		50,593	690			51,283
Net issuance/forfeiture of nonvested shares		(44,595)		44,595
Dividends accrued, $1.84, $1.63 and $1.00 per share for 2016, 2017 and 2018, respectively			(136,497)			(136,497)
Repurchase of common stock				(35,768)		(35,768)
Other comprehensive income (loss)					7,281	7,281
Balance at the end 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 Redeemable Noncontrolling Interest
Net income (loss)						2,930
Net subscription of redeemable noncontrolling interests in sponsored funds						926
Balance at the end of the period at Dec. 31, 2017						14,509
Increase (Decrease) in Stockholders' Equity
Adoption of new accounting principle guidance		3,504	(2,200)			1,304
Net income (loss)			183,588			183,588
Recognition of equity compensation		40,598	1,383			41,981
Net issuance/forfeiture of nonvested shares		(30,744)		30,744
Dividends accrued, $1.84, $1.63 and $1.00 per share for 2016, 2017 and 2018, respectively			(79,768)			(79,768)
Repurchase of common stock				(135,890)		(135,890)
Other comprehensive income (loss)					655	655
Balance at the end 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 Redeemable Noncontrolling Interest
Net income (loss)						(776)
Net redemption of redeemable noncontrolling interests in sponsored funds						(2,270)
Balance at the end of the period at Dec. 31, 2018						$ 11,463
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)