CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Capital Stock	Additional Paid-In Capital	Retained Earnings	Retained Earnings Cumulative-effect adjustment	Accumulated Other Comprehensive Loss
Beginning Balance at Dec. 31, 2017		$ 18,255	$ 128,933	$ 684,540		$ (65,155)
Beginning Balance (Cumulative-effect adjustment for revenue recognition) at Dec. 31, 2017					$ 4,212
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of shares withheld for taxes		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)
Ending Balance (Cumulative-effect adjustment for lease accounting) at Dec. 31, 2018					$ (20)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of shares withheld for taxes		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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of shares withheld for taxes		82	(5,439)
Net income	49,990			49,990
Stock options exercised		0	0
Shares issued for employee stock purchase plan		13	1,347
Shares issued to directors		5	(5)
Shares repurchased	0	0	0
Equity compensation expense			13,532
Other comprehensive income (loss)	27,330					27,330
Ending Balance at Dec. 31, 2020	$ 1,020,755	$ 18,677	$ 147,961	$ 873,692		$ (19,575)