CONDENSED CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Capital Stock	Additional Paid-In Capital	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	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		137	(7,561)
Stock options exercised		10	317
Shares issued for employee stock purchase plan		7	587
Shares issued to directors		10	(10)
Equity compensation expense			6,143
Net income	$ 52,692			52,692
Other comprehensive income (loss)	(3,122)					(3,122)
Balance, end of period at Jun. 30, 2019	897,514	18,559	131,836	829,075		(81,956)
Balance, beginning of period at Mar. 31, 2019		18,546	128,417	804,782	0	(83,423)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of shares withheld for taxes		4	(288)
Stock options exercised		2	40
Shares issued for employee stock purchase plan			7
Shares issued to directors		7	(7)
Equity compensation expense			3,667
Net income	24,293			24,293
Other comprehensive income (loss)	1,467					1,467
Balance, end of period at Jun. 30, 2019	$ 897,514	18,559	131,836	829,075		(81,956)
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201602Member
Balance, beginning of period at Dec. 31, 2019	$ 933,900	18,577	138,526	823,702	0	(46,905)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of shares withheld for taxes		85	(5,114)
Shares issued for employee stock purchase plan		6	658
Shares issued to directors		0
Equity compensation expense			7,022
Net income	27,779			27,779
Other comprehensive income (loss)	(1,754)					(1,754)
Balance, end of period at Jun. 30, 2020	962,582	18,668	141,092	851,481		(48,659)
Balance, beginning of period at Mar. 31, 2020		18,662	137,235	836,961	$ 0	(55,049)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of shares withheld for taxes		6	(38)
Equity compensation expense			3,895
Net income	14,520			14,520
Other comprehensive income (loss)	6,390					6,390
Balance, end of period at Jun. 30, 2020	$ 962,582	$ 18,668	$ 141,092	$ 851,481		$ (48,659)