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, 2016					$ (6,757)
Increase (Decrease) in Stockholders' Equity
Net income (loss)						$ 111,515
Balance at the end of the period at Sep. 30, 2017					60
Balance at the beginning of the period at Jun. 30, 2017					(1,980)
Increase (Decrease) in Stockholders' Equity
Net income (loss)						53,582
Balance at the end of the period at Sep. 30, 2017					60
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
Adoption of new accounting principle guidance | ASU 2016-01			812		(812)
Adoption of new accounting principle guidance | ASU 2018-02			36		(36)
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, $0.75 per share			(60,568)			(60,568)
Repurchase of common stock				(89,018)		(89,018)
Other comprehensive 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					(1,332)
Increase (Decrease) in Stockholders' Equity
Net income (loss)						46,305
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 end of the period at Sep. 30, 2018						$ 16,133