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	$ 73,460
Other comprehensive income (loss)	22,942
Balance, End of Period at Sep. 30, 2017					(69,320)
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	63,199	0	0	63,199	0
Other comprehensive income (loss)	(8,462)	0	0	0	(8,462)
Stock options exercised	734	19	715	0	0
Stock issued to directors	0	12	(12)	0	0
Shares issued for employees stock purchase plan	1,082	12	1,070	0	0
Shares issued for vested restricted stock units, net of cancellations for tax withholding	(6,492)	115	(6,607)	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	8,536	0	8,536	0	0
Balance, End of Period at Sep. 30, 2018	$ 826,383	$ 18,390	$ 129,659	$ 751,951	$ (73,617)