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	3,302				3,302
Employee stock options, restricted stock and employee stock purchase plan	(24)	14	1,319		(350)	(1,007)
Stock-based compensation expense	8,458		8,458
Foreign currency translation	9,481			9,481
Ending balance at Sep. 30, 2017	521,760	1,012	568,743	(53,727)	132,825	(127,093)
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	21,712				21,712
Employee stock options, restricted stock and employee stock purchase plan	774	17	3,005		(90)	(2,158)
Stock-based compensation expense	8,497		8,497
Foreign currency translation	(11,548)			(11,548)
Ending balance at Sep. 30, 2018	$ 560,151	$ 1,063	$ 615,351	$ (64,767)	$ 138,233	$ (129,729)