CONDENSED CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Capital Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Balance, beginning of period at Dec. 31, 2019		$ 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)
Shares issued for employee stock purchase plan		13	1,360
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)
Shares issued for employee stock purchase plan		7	702
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)
Balance, beginning of period at Dec. 31, 2020	1,020,755	18,677	147,961	873,692	(19,575)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of shares withheld for taxes		29	(2,781)
Shares issued for employee stock purchase plan		13	1,550
Shares issued to directors		10	(10)
Equity compensation expense			13,147
Net income	85,002			85,002
Other comprehensive income (loss)	(18,454)				(18,454)
Balance, end of period at Sep. 30, 2021	1,099,261	18,729	159,867	958,694	(38,029)
Balance, beginning of period at Jun. 30, 2021		18,722	154,330	933,565	(29,955)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of shares withheld for taxes		1	(69)
Shares issued for employee stock purchase plan		6	853
Equity compensation expense			4,753
Net income	25,129			25,129
Other comprehensive income (loss)	(8,074)				(8,074)
Balance, end of period at Sep. 30, 2021	$ 1,099,261	$ 18,729	$ 159,867	$ 958,694	$ (38,029)