Condensed Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total		Common stock [Member]	Additional paid-in capital [Member]	Retained 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	657,979				657,979
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	7,575			7,575
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares			34,000
Net issuance of common stock upon exercise of stock options	22,797		$ 5	22,792
Net issuance of common stock upon exercise of stock options, shares			415,000
Share-based compensation	9,460			9,460
Share repurchases, including fees	$ (965,867)		$ (38)	(57,033)	(908,796)
Share repurchases, shares	(3,763,000)		(3,763,000)
Balance at Jun. 30, 2018	$ 384,990		$ 810	1,247,837	(863,657)
Balance (in shares) at Jun. 30, 2018			80,988,000
Balance at Mar. 31, 2018	423,828		$ 823	1,247,366	(824,361)
Balance (in shares) at Mar. 31, 2018			82,268,000
Net income	353,073				353,073
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	3,942			3,942
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	15,939		$ 3	15,936
Net issuance of common stock upon exercise of stock options, shares			278,000
Share-based compensation	4,625			4,625
Share repurchases, including fees	$ (416,417)		$ (16)	(24,032)	(392,369)
Share repurchases, shares	(1,575,000)		(1,575,000)
Balance at Jun. 30, 2018	$ 384,990		$ 810	1,247,837	(863,657)
Balance (in shares) at Jun. 30, 2018			80,988,000
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	$ 674,833				674,833
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	8,053			8,053
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares			25,000
Net issuance of common stock upon exercise of stock options	19,928		$ 2	19,926
Net issuance of common stock upon exercise of stock options, shares			181,000
Share-based compensation	10,328			10,328
Share repurchases, including fees	$ (920,717)		$ (25)	(41,440)	(879,252)
Share repurchases, shares	(2,560,000)		(2,560,000)
Balance at Jun. 30, 2019	$ 144,682		$ 767	1,258,930	(1,115,015)
Balance (in shares) at Jun. 30, 2019	76,690,215		76,690,000
Balance at Mar. 31, 2019	$ 372,365		$ 783	1,268,032	(896,450)
Balance (in shares) at Mar. 31, 2019			78,262,000
Net income	353,681				353,681
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	4,281			4,281
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares			13,000
Net issuance of common stock upon exercise of stock options	7,973			7,973
Net issuance of common stock upon exercise of stock options, shares			48,000
Share-based compensation	5,243			5,243
Share repurchases, including fees	$ (598,861)		$ (16)	(26,599)	(572,246)
Share repurchases, shares	(1,633,000)		(1,633,000)
Balance at Jun. 30, 2019	$ 144,682		$ 767	$ 1,258,930	$ (1,115,015)
Balance (in shares) at Jun. 30, 2019	76,690,215		76,690,000

[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.