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, 2017		$ 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		113	(6,540)
Stock options exercised		18	680
Shares issued for employee stock purchase plan		6	552
Shares issued to directors		11	(11)
Shares repurchased	$ (3,000)	(23)	(2,976)
Equity compensation expense			5,814
Net income	43,465			43,465
Other comprehensive income (loss)	(7,109)				(7,109)
Balance, end of period at Jun. 30, 2018	804,785	18,380	126,452	732,217	(72,264)
Balance, beginning of period at Mar. 31, 2018		18,365	123,104	714,888	(57,334)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of cancellations for tax withholding		0	(20)
Stock options exercised		7	218
Shares issued for employee stock purchase plan		0	17
Shares issued to directors		8	(8)
Shares repurchased	(3,000)	0	0
Equity compensation expense			3,141
Net income	17,329			17,329
Other comprehensive income (loss)	(14,930)				(14,930)
Balance, end of period at Jun. 30, 2018	804,785	18,380	126,452	732,217	(72,264)
Balance, beginning of period 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		137	(7,561)
Stock options exercised		10	317
Shares issued for employee stock purchase plan		7	587
Shares issued to directors		10	(10)
Shares repurchased		0	0
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	(83,423)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of cancellations for tax withholding		4	(288)
Stock options exercised		2	40
Shares issued for employee stock purchase plan		0	7
Shares issued to directors		7	(7)
Shares repurchased		0	0
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)