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	1,024,130				1,024,130
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	10,799			10,799
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares			47,000
Net issuance of common stock upon exercise of stock options	47,351		$ 7	47,344
Net issuance of common stock upon exercise of stock options, shares			692,000
Share-based compensation	14,113			14,113
Share repurchases, including fees	$ (1,251,060)		$ (47)	(71,472)	(1,179,541)
Share repurchases, shares	(4,695,000)		(4,695,000)
Balance at Sep. 30, 2018	$ 498,379		$ 803	1,265,827	(768,251)
Balance (in shares) at Sep. 30, 2018			80,346,000
Balance at Jun. 30, 2018	384,990		$ 810	1,247,837	(863,657)
Balance (in shares) at Jun. 30, 2018			80,988,000
Net income	366,151				366,151
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	3,224			3,224
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	24,554		$ 2	24,552
Net issuance of common stock upon exercise of stock options, shares			277,000
Share-based compensation	4,653			4,653
Share repurchases, including fees	$ (285,193)		$ (9)	(14,439)	(270,745)
Share repurchases, shares	(932,000)		(932,000)
Balance at Sep. 30, 2018	$ 498,379		$ 803	1,265,827	(768,251)
Balance (in shares) at Sep. 30, 2018			80,346,000
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	$ 1,066,126				1,066,126
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	11,576			11,576
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares			36,000
Net issuance of common stock upon exercise of stock options	28,703		$ 3	28,700
Net issuance of common stock upon exercise of stock options, shares			233,000
Share-based compensation	15,540			15,540
Share repurchases, including fees	$ (1,307,983)		$ (36)	(58,335)	(1,249,612)
Share repurchases, shares	(3,585,000)		(3,585,000)
Balance at Sep. 30, 2019	$ 166,219		$ 757	1,259,544	(1,094,082)
Balance (in shares) at Sep. 30, 2019	75,727,781		75,728,000
Balance at Jun. 30, 2019	$ 144,682		$ 767	1,258,930	(1,115,015)
Balance (in shares) at Jun. 30, 2019			76,690,000
Net income	391,293				391,293
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	3,523			3,523
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares			11,000
Net issuance of common stock upon exercise of stock options	8,775		$ 1	8,774
Net issuance of common stock upon exercise of stock options, shares			52,000
Share-based compensation	5,212			5,212
Share repurchases, including fees	$ (387,266)		$ (11)	(16,895)	(370,360)
Share repurchases, shares	(1,025,000)		(1,025,000)
Balance at Sep. 30, 2019	$ 166,219		$ 757	$ 1,259,544	$ (1,094,082)
Balance (in shares) at Sep. 30, 2019	75,727,781		75,728,000

[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.