Condensed Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total		Common stock [Member]	Additional paid-in capital [Member]	Retained earnings (deficit) [Member]
Cumulative effect adjustment from adoption of ASU 2016-09 | Adoption of ASU 2016-09 [Member]	$ 168			$ 434	$ (266)
Balance at Dec. 31, 2016	1,627,136		$ 929	1,336,707	289,500
Balance (in shares) at Dec. 31, 2016			92,852,000
Net income	831,489				831,489
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	10,261			10,261
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes, shares			50,000
Net issuance of common stock upon exercise of stock options	24,788		$ 4	24,784
Net issuance of common stock upon exercise of stock options, shares			483,000
Share-based compensation	13,485			13,485
Share repurchases, including fees	$ (1,893,148)		$ (80)	(117,861)	(1,775,207)
Share repurchases, shares	(8,047,000)		(8,047,000)
Balance at Sep. 30, 2017	$ 614,179		$ 853	1,267,810	(654,484)
Balance (in shares) at Sep. 30, 2017			85,338,000
Balance at Jun. 30, 2017	869,312		$ 880	1,296,674	(428,242)
Balance (in shares) at Jun. 30, 2017			87,999,000
Net income	283,734				283,734
Issuance of common stock under employee benefit plans, net of forfeitures and shares withheld to cover taxes	3,087			3,087
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	4,379			4,379
Net issuance of common stock upon exercise of stock options, shares			65,000
Share-based compensation	4,224			4,224
Share repurchases, including fees	$ (550,557)		$ (27)	(40,554)	(509,976)
Share repurchases, shares	(2,743,000)		(2,743,000)
Balance at Sep. 30, 2017	$ 614,179		$ 853	1,267,810	(654,484)
Balance (in shares) at Sep. 30, 2017			85,338,000
Balance at Dec. 31, 2017	$ 653,046	[1]	$ 843	1,265,043	(612,840)
Balance (in shares) at Dec. 31, 2017	84,302,187		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	80,345,665		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	80,345,665		80,346,000

[1] The balance sheet at December 31, 2017, 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.