CONDENSED CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Capital Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance, Beginning of Period at Dec. 31, 2016					$ (92,262)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 47,928
Other comprehensive income (loss)	16,772
Balance, End of Period at Jun. 30, 2017					(75,490)
Balance, Beginning of Period at Dec. 31, 2017	766,573	$ 18,255	$ 128,933	$ 684,540	(65,155)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	43,465	0	0	43,465	0
Other comprehensive income (loss)	(7,109)	0	0	0	(7,109)
Stock options exercised	698	18	680	0	0
Stock issued to directors	0	11	(11)	0	0
Shares issued for employees stock purchase plan	558	6	552	0	0
Shares issued for vested restricted stock units, net of cancellations for tax withholding	(6,427)	113	(6,540)	0	0
Value of capital stock repurchased	(2,999)	(23)	(2,976)	0	0
Cumulative-effect adjustment of revenue recognition ASC 606	4,212	0	0	4,212	0
Equity compensation expense	5,814	0	5,814	0	0
Balance, End of Period at Jun. 30, 2018	$ 804,785	$ 18,380	$ 126,452	$ 732,217	$ (72,264)