CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Capital Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning Balance at Dec. 31, 2015	$ 584,582	$ 17,957	$ 112,017	$ 543,066	$ (88,458)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	48,283			48,283
Other comprehensive income (loss)	(3,804)				(3,804)
Shares issued for vested restricted stock units, net of cancellations for tax withholding	(1,377)	63	(1,440)
Stock options exercised	4,143	95	4,048
Shares issued for employee stock purchase plan	858	23	835
Shares issued to directors	0	24	(24)
Equity compensation expense	11,275		11,275
Tax adjustments on share-based compensation	(179)		(179)
Shares repurchased	(7,995)	(141)	(7,854)
Ending Balance at Dec. 31, 2016	635,786	18,021	118,678	591,349	(92,262)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	80,459			80,459
Other comprehensive income (loss)	27,107				27,107
Shares issued for vested restricted stock units, net of cancellations for tax withholding	(5,309)	121	(5,430)
Stock options exercised	3,085	83	3,002
Shares issued for employee stock purchase plan	895	15	880
Shares issued to directors	0	15	(15)
Equity compensation expense	11,818		11,818
Cumulative-effect adjustment of change in accounting for share-based compensation	12,732			12,732
Shares repurchased	0		0
Ending Balance at Dec. 31, 2017	766,573	18,255	128,933	684,540	(65,155)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	87,651			87,651
Other comprehensive income (loss)	(13,679)				(13,679)
Shares issued for vested restricted stock units, net of cancellations for tax withholding	(6,600)	117	(6,717)
Stock options exercised	861	22	839
Shares issued for employee stock purchase plan	1,082	12	1,070
Shares issued to directors	0	12	(12)
Equity compensation expense	11,223	0	11,223
Cumulative-effect adjustment of change in accounting for share-based compensation	4,212			4,212
Shares repurchased	(2,999)	(23)	(2,976)
Ending Balance at Dec. 31, 2018	$ 848,324	$ 18,395	$ 132,360	$ 776,403	$ (78,834)