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, 2015	$ 1,961,314	$ 977	$ 1,281,497	$ 678,840
Balance (in shares) at Dec. 31, 2015		97,737,000
Net income	1,037,691			1,037,691
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	12,614	$ 1	12,613
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares		56,000
Net issuance of common stock upon exercise of stock options	47,394	$ 8	47,386
Net issuance of common stock upon exercise of stock options, shares		757,000
Excess tax benefit from share-based compensation	55,994		55,994
Share based compensation	17,566		17,566
Share repurchases, including fees	(1,505,437)	$ (57)	(78,349)	(1,427,031)
Share repurchases, shares		(5,698,000)
Balance at Dec. 31, 2016	1,627,136	$ 929	1,336,707	289,500
Balance (in shares) at Dec. 31, 2016		92,852,000
Cumulative effective adjustment from adoption of ASU 2016-09 | Adoption of ASU 2016-09 [Member]	168		434	(266)
Net income	1,133,804			1,133,804
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	13,466		13,466
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares		66,000
Net issuance of common stock upon exercise of stock options	33,229	$ 7	33,222
Net issuance of common stock upon exercise of stock options, shares		685,000
Share based compensation	17,773		17,773
Share repurchases, including fees	$ (2,172,530)	$ (93)	(136,559)	(2,035,878)
Share repurchases, shares	(9,301,000)	(9,301,000)
Balance at Dec. 31, 2017	$ 653,046	$ 843	1,265,043	(612,840)
Balance (in shares) at Dec. 31, 2017	84,302,187	84,302,000
Net income	$ 1,324,487			1,324,487
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	14,173		14,173
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares		58,000
Net issuance of common stock upon exercise of stock options	57,168	$ 8	57,160
Net issuance of common stock upon exercise of stock options, shares		745,000
Share based compensation	18,806		18,806
Share repurchases, including fees	$ (1,714,013)	$ (61)	(93,119)	(1,620,833)
Share repurchases, shares	(6,061,000)	(6,061,000)
Balance at Dec. 31, 2018	$ 353,667	$ 790	$ 1,262,063	$ (909,186)
Balance (in shares) at Dec. 31, 2018	79,043,919	79,044,000