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, 2016		$ 18,021	$ 118,678	$ 591,349	$ (92,262)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of cancellations for tax withholding		121	(5,430)
Net income	$ 80,459			80,459
Stock options exercised		83	3,002
Shares issued for employee stock purchase plan		15	880
Shares issued to directors		15	(15)
Shares repurchased	0	0	0
Equity compensation expense			11,818
Other comprehensive income (loss)	27,107				27,107
Ending Balance at Dec. 31, 2017	766,573	18,255	128,933	684,540	(65,155)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of cancellations for tax withholding		117	(6,717)
Net income	87,651			87,651
Stock options exercised		22	839
Shares issued for employee stock purchase plan		12	1,070
Shares issued to directors		12	(12)
Shares repurchased	(2,999)	(23)	(2,976)
Equity compensation expense			11,223
Other comprehensive income (loss)	(13,679)				(13,679)
Ending Balance at Dec. 31, 2018	848,324	18,395	132,360	776,403	(78,834)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of cancellations for tax withholding		144	(7,694)
Net income	47,319			47,319
Stock options exercised		11	333
Shares issued for employee stock purchase plan		15	1,234
Shares issued to directors		12	(12)
Shares repurchased	0	0	0
Equity compensation expense			12,305
Other comprehensive income (loss)	31,929				31,929
Ending Balance at Dec. 31, 2019	$ 933,900	$ 18,577	$ 138,526	$ 823,702	$ (46,905)