Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)	Total
Balance (in shares) at Dec. 31, 2017	32,048
Balance at Dec. 31, 2017	$ 32	$ 556,147	$ (204,745)	$ 2,176	$ 353,610
Increase (Decrease) in Stockholders' Equity
Net income			13,915		13,915
Foreign currency translation adjustments				1,021	1,021
Change in pension obligation				30	30
Exercise of stock options (in shares)	75
Exercise of stock options		446			446
Issuance of restricted common shares (in shares)	19
Issuance of restricted common shares		(227)			(227)
Stock-based compensation expense		1,132			1,132
Balance (in shares) at Mar. 31, 2018	32,142
Balance at Mar. 31, 2018	$ 32	557,498	(189,230)	3,227	371,527
Balance (in shares) at Dec. 31, 2017	32,048
Balance at Dec. 31, 2017	$ 32	556,147	(204,745)	2,176	353,610
Increase (Decrease) in Stockholders' Equity
Net income					37,422
Foreign currency translation adjustments					(1,584)
Change in pension obligation					90
Balance (in shares) at Sep. 30, 2018	32,398
Balance at Sep. 30, 2018	$ 32	562,083	(165,723)	682	397,074
Increase (Decrease) in Stockholders' Equity
Adjustment to Retained Earnings upon ASC 606 Adoption			1,600		1,600
Balance (in shares) at Mar. 31, 2018	32,142
Balance at Mar. 31, 2018	$ 32	557,498	(189,230)	3,227	371,527
Increase (Decrease) in Stockholders' Equity
Net income			14,669		14,669
Foreign currency translation adjustments				(2,321)	(2,321)
Change in pension obligation				30	30
Exercise of stock options (in shares)	46
Exercise of stock options		376			376
Issuance of shares under Employee Stock Purchase Plan (in shares)	26
Issuance of shares under Employee Stock Purchase Plan		515			515
Issuance of restricted common shares (in shares)	117
Issuance of restricted common shares		(1,161)			(1,161)
Stock-based compensation expense		1,979			1,979
Balance (in shares) at Jun. 30, 2018	32,331
Balance at Jun. 30, 2018	$ 32	559,207	(174,561)	936	385,614
Increase (Decrease) in Stockholders' Equity
Net income			8,838		8,838
Foreign currency translation adjustments				(284)	(284)
Change in pension obligation				30	30
Exercise of stock options (in shares)	66
Exercise of stock options		467			467
Issuance of restricted common shares (in shares)	1
Issuance of restricted common shares		(6)			(6)
Stock-based compensation expense		2,415			2,415
Balance (in shares) at Sep. 30, 2018	32,398
Balance at Sep. 30, 2018	$ 32	562,083	(165,723)	682	397,074
Balance (in shares) at Dec. 31, 2018	32,558
Balance at Dec. 31, 2018	$ 33	565,116	(157,260)	448	408,337
Increase (Decrease) in Stockholders' Equity
Net income			6,062		6,062
Foreign currency translation adjustments				(495)	(495)
Change in pension obligation				30	30
Exercise of stock options (in shares)	288
Exercise of stock options		1,828			1,828
Issuance of restricted common shares (in shares)	35
Issuance of restricted common shares		(281)			(281)
Stock-based compensation expense		1,672			1,672
Balance (in shares) at Mar. 31, 2019	32,881
Balance at Mar. 31, 2019	$ 33	568,335	(151,198)	(17)	417,153
Balance (in shares) at Dec. 31, 2018	32,558
Balance at Dec. 31, 2018	$ 33	565,116	(157,260)	448	408,337
Increase (Decrease) in Stockholders' Equity
Net income					7,322
Foreign currency translation adjustments					(1,847)
Change in pension obligation					89
Balance (in shares) at Sep. 30, 2019	32,397
Balance at Sep. 30, 2019	$ 32	559,063	(149,938)	(1,310)	407,847
Balance (in shares) at Mar. 31, 2019	32,881
Balance at Mar. 31, 2019	$ 33	568,335	(151,198)	(17)	417,153
Increase (Decrease) in Stockholders' Equity
Net income			556		556
Foreign currency translation adjustments				(219)	(219)
Change in pension obligation				29	29
Exercise of stock options (in shares)	104
Exercise of stock options		567			567
Issuance of shares under Employee Stock Purchase Plan (in shares)	32
Issuance of shares under Employee Stock Purchase Plan		479			479
Issuance of restricted common shares (in shares)	165
Issuance of restricted common shares		(1,310)			(1,310)
Stock-based compensation expense		2,246			2,246
Repurchase of common stock (in shares)	(885)
Repurchase of common stock	$ (1)	(14,034)			(14,035)
Balance (in shares) at Jun. 30, 2019	32,297
Balance at Jun. 30, 2019	$ 32	556,283	(150,642)	(207)	405,466
Increase (Decrease) in Stockholders' Equity
Net income			704		704
Foreign currency translation adjustments				(1,132)	(1,132)
Change in pension obligation				29	29
Exercise of stock options (in shares)	97
Exercise of stock options		642			642
Issuance of restricted common shares (in shares)	3
Issuance of restricted common shares		(11)			(11)
Stock-based compensation expense		2,149			2,149
Balance (in shares) at Sep. 30, 2019	32,397
Balance at Sep. 30, 2019	$ 32	$ 559,063	$ (149,938)	$ (1,310)	$ 407,847