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		115	(6,607)
Stock options exercised		19	715
Shares issued for employee stock purchase plan		12	1,070
Shares issued to directors		12	(12)
Shares repurchased	$ (3,000)	(23)	(2,976)
Equity compensation expense			8,536
Net income	63,199			63,199
Other comprehensive income (loss)	(8,462)				(8,462)
Balance, end of period at Sep. 30, 2018	826,383	18,390	129,659	751,951	(73,617)
Balance, beginning of period at Jun. 30, 2018		18,380	126,452	732,217	(72,264)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for vested restricted stock units, net of cancellations for tax withholding		2	(67)
Stock options exercised		1	35
Shares issued for employee stock purchase plan		6	518
Shares issued to directors		1	(1)
Shares repurchased		0	0
Equity compensation expense			2,722
Net income	19,734			19,734
Other comprehensive income (loss)	(1,353)				(1,353)
Balance, end of period at Sep. 30, 2018	826,383	18,390	129,659	751,951	(73,617)
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		139	(7,448)
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
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 cancellations for tax withholding		2	113
Stock options exercised		1	16
Shares issued for employee stock purchase plan		8	647
Shares issued to directors		2	(2)
Shares repurchased		0	0
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)