CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY Statement - USD ($) $ 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, 2018	$ 874,659	$ 677	$ (8,458)	$ 672,095	$ 210,345
Common stock issued beginning of period (shares) at Dec. 31, 2018		67,708,000
Treasury stock beginning of period (shares) at Dec. 31, 2018			478,000
Stockholders' Equity [Abstract]
Exercise of stock options	132	$ 0		132
Exercise of stock options (shares)		8,000
Shares vested for long-term incentive plan	0	$ 3		(3)
Shares vested for long-term incentive plan (shares)		290,000
Repurchases of common stock under share repurchase program	(15,709)		$ (15,709)
Repurchases of common stock under share repurchase program (shares)			920,000
Repurchases of common stock equity related to equity award activity	(1,330)		$ (1,330)
Repurchases of common stock related to equity award activity (shares)			74,000
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		68,006,000
Treasury stock end of period (shares) at Mar. 31, 2019			1,472,000
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		67,708,000
Treasury stock beginning of period (shares) at Dec. 31, 2018			478,000
Stockholders' Equity [Abstract]
Repurchases of common stock under share repurchase program	$ (16,400)
Repurchases of common stock under share repurchase program (shares)	1,000,000.0
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		68,266,000
Treasury stock end of period (shares) at Sep. 30, 2019			1,526,000
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		68,006,000
Treasury stock beginning of period (shares) at Mar. 31, 2019			1,472,000
Stockholders' Equity [Abstract]
Exercise of stock options	529	$ 1		528
Exercise of stock options (shares)		76,000
Shares vested for long-term incentive plan	0	$ 1		(1)
Shares vested for long-term incentive plan (shares)		73,000
Repurchases of common stock under share repurchase program	(737)		$ (737)
Repurchases of common stock under share repurchase program (shares)			41,000
Repurchases of common stock equity related to equity award activity	(137)		$ (137)
Repurchases of common stock related to equity award activity (shares)			6,000
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		68,155,000
Treasury stock end of period (shares) at Jun. 30, 2019			1,519,000
Stockholders' Equity [Abstract]
Exercise of stock options	1,533	$ 1		1,532
Exercise of stock options (shares)		84,000
Shares vested for long-term incentive plan	$ 0	$ 0		0
Shares vested for long-term incentive plan (shares)		27,000
Repurchases of common stock under share repurchase program (shares)	0
Repurchases of common stock equity related to equity award activity	$ (182)		$ (182)
Repurchases of common stock related to equity award activity (shares)			7,000
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		68,266,000
Treasury stock end of period (shares) at Sep. 30, 2019			1,526,000
Stockholders' equity beginning of period at Dec. 31, 2019	$ 981,512	$ 683	$ (26,616)	687,255	320,190
Common stock issued beginning of period (shares) at Dec. 31, 2019	68,300,000	68,306,000
Treasury stock beginning of period (shares) at Dec. 31, 2019	1,500,000		1,528,000
Stockholders' Equity [Abstract]
Exercise of stock options	$ 206	$ 0		206
Exercise of stock options (shares)		13,000
Shares vested for long-term incentive plan	0	$ 4		(4)
Shares vested for long-term incentive plan (shares)		402,000
Repurchases of common stock under share repurchase program	(1,416)		$ (1,416)
Repurchases of common stock under share repurchase program (shares)			87,000
Repurchases of common stock equity related to equity award activity	(2,441)		$ (2,441)
Repurchases of common stock related to equity award activity (shares)			122,000
Stock compensation expense	3,170			3,170
Net income	22,029				22,029
Stockholders' equity end of period at Mar. 31, 2020	1,003,060	$ 687	$ (30,473)	690,627	342,219
Common stock issued end of period (shares) at Mar. 31, 2020		68,721,000
Treasury stock end of period (shares) at Mar. 31, 2020			1,737,000
Stockholders' equity beginning of period at Dec. 31, 2019	$ 981,512	$ 683	$ (26,616)	687,255	320,190
Common stock issued beginning of period (shares) at Dec. 31, 2019	68,300,000	68,306,000
Treasury stock beginning of period (shares) at Dec. 31, 2019	1,500,000		1,528,000
Stockholders' Equity [Abstract]
Repurchases of common stock under share repurchase program	$ (1,400)
Repurchases of common stock under share repurchase program (shares)	100,000
Net income	$ 110,546
Stockholders' equity end of period at Sep. 30, 2020	$ 1,102,507	$ 690	$ (30,964)	702,045	430,736
Common stock issued end of period (shares) at Sep. 30, 2020	69,000,000.0	69,028,000
Treasury stock end of period (shares) at Sep. 30, 2020	1,800,000		1,753,000
Stockholders' equity beginning of period at Mar. 31, 2020	$ 1,003,060	$ 687	$ (30,473)	690,627	342,219
Common stock issued beginning of period (shares) at Mar. 31, 2020		68,721,000
Treasury stock beginning of period (shares) at Mar. 31, 2020			1,737,000
Stockholders' Equity [Abstract]
Exercise of stock options	159	$ 0		159
Exercise of stock options (shares)		9,000
Shares vested for long-term incentive plan	0	$ 1		(1)
Shares vested for long-term incentive plan (shares)		75,000
Repurchases of common stock equity related to equity award activity	(105)		$ (105)
Repurchases of common stock related to equity award activity (shares)			6,000
Stock compensation expense	3,328			3,328
Net income	43,622				43,622
Stockholders' equity end of period at Jun. 30, 2020	1,050,064	$ 688	$ (30,578)	694,113	385,841
Common stock issued end of period (shares) at Jun. 30, 2020		68,805,000
Treasury stock end of period (shares) at Jun. 30, 2020			1,743,000
Stockholders' Equity [Abstract]
Exercise of stock options	3,322	$ 2		3,320
Exercise of stock options (shares)		196,000
Shares vested for long-term incentive plan	$ 0	$ 0		0
Shares vested for long-term incentive plan (shares)		27,000
Repurchases of common stock under share repurchase program (shares)	0
Repurchases of common stock equity related to equity award activity	$ (386)		$ (386)
Repurchases of common stock related to equity award activity (shares)			10,000
Stock compensation expense	4,612			4,612
Net income	44,895				44,895
Stockholders' equity end of period at Sep. 30, 2020	$ 1,102,507	$ 690	$ (30,964)	$ 702,045	$ 430,736
Common stock issued end of period (shares) at Sep. 30, 2020	69,000,000.0	69,028,000
Treasury stock end of period (shares) at Sep. 30, 2020	1,800,000		1,753,000