Condensed Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total		Common stock [Member]	Additional paid-in capital [Member]	Retained earnings (deficit) [Member]
Balance at Dec. 31, 2017	$ 653,046		$ 843	$ 1,265,043	$ (612,840)
Balance (in shares) at Dec. 31, 2017			84,302,000
Net income	304,906				304,906
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	3,633			3,633
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares			17,000
Net issuance of common stock upon exercise of stock options	6,858		$ 2	6,856
Net issuance of common stock upon exercise of stock options, shares			137,000
Share-based compensation	4,835			4,835
Share repurchases, including fees	$ (549,450)		$ (22)	(33,001)	(516,427)
Share repurchases, shares	(2,188,000)		(2,188,000)
Balance at Mar. 31, 2018	$ 423,828		$ 823	1,247,366	(824,361)
Balance (in shares) at Mar. 31, 2018			82,268,000
Cumulative effect adjustment from adoption of ASU 2016-02 | Adoption of ASU 2016-02 [Member]	(1,410)				(1,410)
Balance at Dec. 31, 2018	$ 353,667	[1]	$ 790	1,262,063	(909,186)
Balance (in shares) at Dec. 31, 2018	79,043,919		79,044,000
Net income	$ 321,152				321,152
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	3,772			3,772
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares			12,000
Net issuance of common stock upon exercise of stock options	11,955		$ 2	11,953
Net issuance of common stock upon exercise of stock options, shares			133,000
Share-based compensation	5,085			5,085
Share repurchases, including fees	$ (321,856)		$ (9)	(14,841)	(307,006)
Share repurchases, shares	(927,000)		(927,000)
Balance at Mar. 31, 2019	$ 372,365		$ 783	$ 1,268,032	$ (896,450)
Balance (in shares) at Mar. 31, 2019	78,262,099		78,262,000

[1] The balance sheet at December 31, 2018, has been derived from the audited consolidated financial statements at that date but does not include all of the information and footnotes required by United States generally accepted accounting principles for complete financial statements.