Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Paid-In Capital	Accumulated Other Comprehensive Loss	Retained Earnings	Treasury Stock
Beginning balance at Dec. 31, 2017	$ 547,480	$ 1,046	$ 603,849	$ (53,219)	$ 123,375	$ (127,571)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	18,068				18,068
Employee stock options, restricted stock and employee stock purchase plan	969	15	2,970		(90)	(1,926)
Stock-based compensation expense	4,848		4,848
Foreign currency translation, net of tax	(9,878)			(9,878)
Ending balance at Jun. 30, 2018	554,723	1,061	611,667	(63,097)	134,589	(129,497)
Beginning balance at Mar. 31, 2018	550,215	1,046	606,491	(53,885)	123,743	(127,180)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	10,846				10,846
Employee stock options, restricted stock and employee stock purchase plan	315	15	2,617			(2,317)
Stock-based compensation expense	2,559		2,559
Foreign currency translation, net of tax	(9,212)			(9,212)
Ending balance at Jun. 30, 2018	554,723	1,061	611,667	(63,097)	134,589	(129,497)
Beginning balance at Dec. 31, 2018	569,681	1,064	617,276	(67,673)	148,802	(129,788)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	5,588				5,588
Employee stock options, restricted stock and employee stock purchase plan	(277)	3	(5,524)		(995)	6,239
Stock-based compensation expense	6,874		6,874
Treasury shares purchased at cost	(15,537)					(15,537)
Foreign currency translation, net of tax	(200)			(200)
Ending balance at Jun. 30, 2019	566,129	1,067	618,626	(67,873)	153,395	(139,086)
Beginning balance at Mar. 31, 2019	569,788	1,064	622,554	(69,594)	150,084	(134,320)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	4,306				4,306
Employee stock options, restricted stock and employee stock purchase plan	(1,067)	3	(5,833)		(995)	5,758
Stock-based compensation expense	1,905		1,905
Treasury shares purchased at cost	(10,524)					(10,524)
Foreign currency translation, net of tax	1,721			1,721
Ending balance at Jun. 30, 2019	$ 566,129	$ 1,067	$ 618,626	$ (67,873)	$ 153,395	$ (139,086)