CONDENSED CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Capital Stock	Additional Paid-In Capital	Retained Earnings	Retained Earnings Cumulative-effect adjustment for lease accounting	Accumulated Other Comprehensive Loss
Balance, beginning of period at Dec. 31, 2018		$ 18,395	$ 132,360	$ 776,403	$ (20)	$ (78,834)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of shares withheld for taxes		139	(7,448)
Stock options exercised		11	333
Shares issued for employee stock purchase plan		15	1,234
Shares issued to directors		12	(12)
Equity compensation expense			9,294
Net income	$ 76,079			76,079
Other comprehensive income (loss)	(13,327)					(13,327)
Balance, end of period at Sep. 30, 2019	914,634	18,572	135,761	852,462		(92,161)
Balance, beginning of period at Jun. 30, 2019		18,559	131,836	829,075		(81,956)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of shares withheld for taxes		2	113
Stock options exercised		1	16
Shares issued for employee stock purchase plan		8	647
Shares issued to directors		2	(2)
Equity compensation expense			3,151
Net income	23,387			23,387
Other comprehensive income (loss)	(10,205)					(10,205)
Balance, end of period at Sep. 30, 2019	914,634	18,572	135,761	852,462		(92,161)
Balance, beginning of period 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		86	(5,193)
Stock options exercised		0	0
Shares issued for employee stock purchase plan		13	1,360
Shares issued to directors		0	0
Equity compensation expense			10,317
Net income	34,785			34,785
Other comprehensive income (loss)	11,756					11,756
Balance, end of period at Sep. 30, 2020	987,024	18,676	145,010	858,487		(35,149)
Balance, beginning of period at Jun. 30, 2020		18,668	141,092	851,481		(48,659)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of shares withheld for taxes		1	(79)
Stock options exercised		0	0
Shares issued for employee stock purchase plan		7	702
Shares issued to directors		0	0
Equity compensation expense			3,295
Net income	7,006			7,006
Other comprehensive income (loss)	13,510					13,510
Balance, end of period at Sep. 30, 2020	$ 987,024	$ 18,676	$ 145,010	$ 858,487		$ (35,149)