CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY Statement - USD ($) shares in Thousands, $ in Thousands	Total	Common stock [Member]	Treasury stock [Member]	Additional paid-in capital [Member]	Retained earnings [Member]
Stockholders' equity, beginning of period at Dec. 31, 2015	$ 628,932	$ 654	$ (426)	$ 626,402	$ 2,302
Stockholders' Equity [Abstract]
Issuance of common stock, net of offering costs	13,776	8		13,768
Exercise of stock options	1,301	2		1,299
Shares vested for long-term incentive plan	0	4		(4)
Repurchases of common stock related to equity award activity	(2,023)		(2,023)
Share withholdings made in satisfaction of exercise price	0		(80)	80
Stock compensation expense	7,252			7,252
Other	483			483
Net income	30,880				30,880
Stockholders' equity, end of period at Dec. 31, 2016	680,601	$ 668	$ (2,529)	649,280	33,182
Common stock outstanding, beginning of period (in shares) at Dec. 31, 2015		65,360
Treasury shares, beginning of period (in shares) at Dec. 31, 2015			25
Stockholders Equity, Common Shares [Abstract]
Issuance of common stock (in shares)		855
Exercise of stock options (in shares)		175
Shares vested for long-term incentive plan		424
Repurchases of common stock related to equity award activity (in shares)			119
Share withholdings made in satisfaction of exercise price (shares)			4
Common stock outstanding, end of period (in shares) at Dec. 31, 2016		66,814
Treasury shares, end of period (in shares) at Dec. 31, 2016			148
Stockholders' Equity [Abstract]
Exercise of stock options	3,396	$ 3		3,393
Shares vested for long-term incentive plan	0	2		(2)
Repurchases of common stock related to equity award activity	(1,292)		$ (1,292)
Stock compensation expense	6,769			6,769
Net income	57,425				57,425
Stockholders' equity, end of period at Dec. 31, 2017	$ 746,899	$ 673	$ (3,821)	659,440	90,607
Stockholders Equity, Common Shares [Abstract]
Exercise of stock options (in shares)		260
Shares vested for long-term incentive plan		212
Repurchases of common stock related to equity award activity (in shares)			59
Common stock outstanding, end of period (in shares) at Dec. 31, 2017	67,100	67,286
Treasury shares, end of period (in shares) at Dec. 31, 2017	200		207
Stockholders' Equity [Abstract]
Exercise of stock options	$ 1,327	$ 1		1,326
Shares vested for long-term incentive plan	0	3		(3)
Repurchases of common stock under share repurchase program	(2,891)		$ (2,891)
Repurchases of common stock related to equity award activity	(1,729)		(1,729)
Share withholdings made in satisfaction of exercise price	0		(17)	17
Stock compensation expense	11,315			11,315
Net income	119,738				119,738
Stockholders' equity, end of period at Dec. 31, 2018	$ 874,659	$ 677	$ (8,458)	$ 672,095	$ 210,345
Stockholders Equity, Common Shares [Abstract]
Exercise of stock options (in shares)		96
Shares vested for long-term incentive plan		326
Repurchases of common stock under share repurchase program (in shares)			182
Repurchases of common stock related to equity award activity (in shares)			88
Share withholdings made in satisfaction of exercise price (shares)			1
Common stock outstanding, end of period (in shares) at Dec. 31, 2018	67,200	67,708
Treasury shares, end of period (in shares) at Dec. 31, 2018	500		478