CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Capital Stock/Capital Shares	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Dec. 31, 2014	$ 587,281	$ 18,404	$ 137,225	$ 496,746	$ (65,094)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	46,320			46,320
Other comprehensive income (loss)	(23,364)				(23,364)
Stock options exercised	6,967	175	6,792
Stock issued to directors	0	16	(16)
Shares issued for employees stock purchase plan	727	13	714
Shares issued for vested restricted stock units, net of cancellations for tax withholding	(2,740)	77	(2,817)
Shares repurchased	(39,993)	(728)	(39,265)
Tax shortfalls on share-based compensation	(259)		(259)
Equity compensation expense	9,643		9,643
Ending 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)
Stock options exercised	4,143	95	4,048
Stock issued to directors	0	24	(24)
Shares issued for employees stock purchase plan	858	23	835
Shares issued for vested restricted stock units, net of cancellations for tax withholding	(1,377)	63	(1,440)
Shares repurchased	(7,995)	(141)	(7,854)
Tax adjustments on share-based compensation	(179)		(179)
Equity compensation expense	11,275		11,275
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
Stock options exercised	3,085	83	3,002
Stock issued to directors	0	15	(15)
Shares issued for employees stock purchase plan	895	15	880
Shares issued for vested restricted stock units, net of cancellations for tax withholding	(5,309)	121	(5,430)
Shares repurchased	0
Tax adjustments on share-based compensation	12,732			12,732
Equity compensation expense	11,818		11,818
Ending Balance at Dec. 31, 2017	$ 766,573	$ 18,255	$ 128,933	$ 684,540	$ (65,155)