Statements of Shareholders’ Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Treasury Stock	Retained Earnings (Accumulated Deficit)	Accumulated Other Comprehensive Income (Loss)
Balance at Dec. 31, 2014	$ 1,003,075	$ 7	$ 526,173	$ (3,921)	$ 481,534	$ (718)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	9,222		9,222
Repurchase of common stock	(112,261)			(112,261)
Proceeds from options exercised	32		32
Excess tax benefits from share-based compensation	8,850		8,850
Changes in comprehensive income	(828)					(828)
Net income	317,220				317,220
Balance at Dec. 31, 2015	1,225,310	7	544,277	(116,182)	798,754	(1,546)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	7,105		7,105
Repurchase of common stock	(102,510)			(102,510)
Proceeds from options exercised	92		92
Excess tax deficiency from share-based compensation	(470)		(470)
Changes in comprehensive income	201					201
Net income	264,879				264,879
Balance at Dec. 31, 2016	1,394,607	7	551,004	(218,692)	1,063,633	(1,345)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	8,522		8,522
Repurchase of common stock	(46,580)			(46,580)
Proceeds from options exercised	45		45
Changes in comprehensive income	(119)					(119)
Retirement of treasury stock	0		(199,418)	199,418
Net income	420,606				420,606
Balance at Dec. 31, 2017	$ 1,777,081	$ 7	$ 360,153	$ (65,854)	$ 1,484,239	$ (1,464)