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, 2017	$ 746,899	$ 673	$ (3,821)	$ 659,440	$ 90,607
Common stock issued beginning of period (shares) at Dec. 31, 2017		67,286
Treasury stock beginning of period (shares) at Dec. 31, 2017			207
Stockholders' Equity [Abstract]
Exercise of stock options	588	$ 1		587
Exercise of stock options (shares)		55
Shares vested for long-term incentive plan	0	$ 1		(1)
Shares vested for long-term incentive plan (shares)		155
Repurchases of common stock equity related to equity award activity	(1,046)		$ (1,046)
Repurchases of common stock related to equity award activity (shares)			53
Share withholdings made in satisfaction of exercise price	0		$ (17)	17
Share withholdings made in satisfaction of exercise price (shares)			1
Stock compensation expense	1,775			1,775
Net income	15,359				15,359
Stockholders' equity end of period at Mar. 31, 2018	763,575	$ 675	$ (4,884)	661,818	105,966
Common stock issued end of period (shares) at Mar. 31, 2018		67,496
Treasury stock end of period (shares) at Mar. 31, 2018			261
Stockholders' equity beginning of period at Dec. 31, 2017	746,899	$ 673	$ (3,821)	659,440	90,607
Common stock issued beginning of period (shares) at Dec. 31, 2017		67,286
Treasury stock beginning of period (shares) at Dec. 31, 2017			207
Stockholders' Equity [Abstract]
Net income	91,622
Stockholders' equity end of period at Sep. 30, 2018	846,658	$ 676	$ (5,208)	668,961	182,229
Common stock issued end of period (shares) at Sep. 30, 2018		67,633
Treasury stock end of period (shares) at Sep. 30, 2018			276
Stockholders' equity beginning of period at Mar. 31, 2018	763,575	$ 675	$ (4,884)	661,818	105,966
Common stock issued beginning of period (shares) at Mar. 31, 2018		67,496
Treasury stock beginning of period (shares) at Mar. 31, 2018			261
Stockholders' Equity [Abstract]
Exercise of stock options	44	$ 0		44
Exercise of stock options (shares)		2
Shares vested for long-term incentive plan	0	$ 1		(1)
Shares vested for long-term incentive plan (shares)		65
Repurchases of common stock equity related to equity award activity	(103)		$ (103)
Repurchases of common stock related to equity award activity (shares)			5
Stock compensation expense	3,141			3,141
Net income	40,405				40,405
Stockholders' equity end of period at Jun. 30, 2018	807,062	$ 676	$ (4,987)	665,002	146,371
Common stock issued end of period (shares) at Jun. 30, 2018		67,563
Treasury stock end of period (shares) at Jun. 30, 2018			266
Stockholders' Equity [Abstract]
Exercise of stock options	649	$ 0		649
Exercise of stock options (shares)		35
Shares vested for long-term incentive plan	0	$ 0
Shares vested for long-term incentive plan (shares)		35
Repurchases of common stock equity related to equity award activity	(221)		$ (221)
Repurchases of common stock related to equity award activity (shares)			10
Stock compensation expense	3,310			3,310
Net income	35,858				35,858
Stockholders' equity end of period at Sep. 30, 2018	846,658	$ 676	$ (5,208)	668,961	182,229
Common stock issued end of period (shares) at Sep. 30, 2018		67,633
Treasury stock end of period (shares) at Sep. 30, 2018			276
Stockholders' equity beginning of period at Dec. 31, 2018	$ 874,659	$ 677	$ (8,458)	672,095	210,345
Common stock issued beginning of period (shares) at Dec. 31, 2018	67,700	67,708
Treasury stock beginning of period (shares) at Dec. 31, 2018	500		478
Stockholders' Equity [Abstract]
Exercise of stock options	$ 132	$ 0		132
Exercise of stock options (shares)		8
Shares vested for long-term incentive plan	0	$ 3		(3)
Shares vested for long-term incentive plan (shares)		290
Repurchases of common stock under share repurchase program	(15,709)		$ (15,709)
Repurchases of common stock under share repurchase program (shares)			920
Repurchases of common stock equity related to equity award activity	(1,330)		$ (1,330)
Repurchases of common stock related to equity award activity (shares)			74
Stock compensation expense	2,915			2,915
Net income	20,350				20,350
Stockholders' equity end of period at Mar. 31, 2019	881,017	$ 680	$ (25,497)	675,139	230,695
Common stock issued end of period (shares) at Mar. 31, 2019		68,006
Treasury stock end of period (shares) at Mar. 31, 2019			1,472
Stockholders' equity beginning of period at Dec. 31, 2018	$ 874,659	$ 677	$ (8,458)	672,095	210,345
Common stock issued beginning of period (shares) at Dec. 31, 2018	67,700	67,708
Treasury stock beginning of period (shares) at Dec. 31, 2018	500		478
Stockholders' Equity [Abstract]
Repurchases of common stock under share repurchase program	$ (16,400)
Repurchases of common stock under share repurchase program (shares)	1,000
Net income	$ 89,646
Stockholders' equity end of period at Sep. 30, 2019	$ 957,581	$ 683	$ (26,553)	683,460	299,991
Common stock issued end of period (shares) at Sep. 30, 2019	68,300	68,266
Treasury stock end of period (shares) at Sep. 30, 2019	1,500		1,526
Stockholders' equity beginning of period at Mar. 31, 2019	$ 881,017	$ 680	$ (25,497)	675,139	230,695
Common stock issued beginning of period (shares) at Mar. 31, 2019		68,006
Treasury stock beginning of period (shares) at Mar. 31, 2019			1,472
Stockholders' Equity [Abstract]
Exercise of stock options	529	$ 1		528
Exercise of stock options (shares)		76
Shares vested for long-term incentive plan	0	$ 1		(1)
Shares vested for long-term incentive plan (shares)		73
Repurchases of common stock under share repurchase program	(737)		$ (737)
Repurchases of common stock under share repurchase program (shares)			41
Repurchases of common stock equity related to equity award activity	(137)		$ (137)
Repurchases of common stock related to equity award activity (shares)			6
Stock compensation expense	3,248			3,248
Net income	35,699				35,699
Stockholders' equity end of period at Jun. 30, 2019	919,619	$ 682	$ (26,371)	678,914	266,394
Common stock issued end of period (shares) at Jun. 30, 2019		68,155
Treasury stock end of period (shares) at Jun. 30, 2019			1,519
Stockholders' Equity [Abstract]
Exercise of stock options	1,533	$ 1		1,532
Exercise of stock options (shares)		84
Shares vested for long-term incentive plan	0	$ 0
Shares vested for long-term incentive plan (shares)		27
Repurchases of common stock equity related to equity award activity	(182)		$ (182)
Repurchases of common stock related to equity award activity (shares)			7
Stock compensation expense	3,014			3,014
Net income	33,597				33,597
Stockholders' equity end of period at Sep. 30, 2019	$ 957,581	$ 683	$ (26,553)	$ 683,460	$ 299,991
Common stock issued end of period (shares) at Sep. 30, 2019	68,300	68,266
Treasury stock end of period (shares) at Sep. 30, 2019	1,500		1,526