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, 2016	$ 500,543	$ 998	$ 558,966	$ (63,208)	$ 129,873	$ (126,086)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	649				649
Employee stock options, restricted stock and employee stock purchase plan	(586)	11	728		(237)	(1,088)
Stock-based compensation expense	5,874		5,874
Foreign currency translation	7,824			7,824
Ending balance at Jun. 30, 2017	514,304	1,009	565,568	(55,384)	130,285	(127,174)
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]
Cumulative effect of accounting changes	(6,764)				(6,764)
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	(9,878)			(9,878)
Ending balance at Jun. 30, 2018	$ 554,723	$ 1,061	$ 611,667	$ (63,097)	$ 134,589	$ (129,497)