Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Earnings [Member]
Beginning Balance at Dec. 31, 2014	$ 303,373	$ 44	$ 288,393	$ 0	$ (3,060)	$ 17,996
Beginning Balance, Shares at Dec. 31, 2014		44,069,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	16,736					16,736
Other comprehensive income (loss), net of tax	(2,281)				(2,281)
Capital contribution from Lone Star Funds	29,750		29,750
Purchase of treasury shares	(48,479)			(48,479)
Purchase of treasury shares ( in shares)		(2,395,049)
Stock option exercise	873		873
Stock option exercise, shares		62,331
Share-based compensation	801		801
Share-based compensation, shares		13,749
Other	(7)				0	(7)
Ending Balance at Dec. 31, 2015	300,766	$ 44	319,817	(48,479)	(5,341)	34,725
Ending Balance, Shares at Dec. 31, 2015		41,750,031
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	44,024					44,024
Other comprehensive income (loss), net of tax	1,932				1,932
Purchase of treasury shares	(40,277)			(40,277)
Purchase of treasury shares ( in shares)		(2,104,606)
Stock option exercise	20		20
Stock option exercise, shares		1,463
Share-based compensation	2,288		2,288
Share-based compensation, shares		33,954
Employee stock purchase program, shares		10,873
Employee stock purchase program	259		259
Ending Balance at Dec. 31, 2016	309,012	$ 44	322,384	(88,756)	(3,409)	78,749
Ending Balance, Shares at Dec. 31, 2016		39,691,715
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	59,848					59,848
Other comprehensive income (loss), net of tax	760				760
Purchase of treasury shares	(54,601)			(54,601)
Purchase of treasury shares ( in shares)		(2,289,162)
Stock option exercise	230		230
Stock option exercise, shares		16,594
Share-based compensation	2,544		2,544
Share-based compensation, shares		93,470
Employee stock purchase program, shares		20,342
Employee stock purchase program	233		233
Ending Balance at Dec. 31, 2017	$ 318,026	$ 44	$ 325,391	$ (143,357)	$ (2,649)	$ 138,597
Ending Balance, Shares at Dec. 31, 2017		37,532,959