Interim Consolidated Statements of Shareholders' Equity and Comprehensive Income (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning balance at Dec. 31, 2016	$ 434,943	$ 448	$ 730,556	$ (3,006,771)	$ 3,065,708	$ (354,998)
Beginning balance, shares at Dec. 31, 2016		26,020,234
Stock Issued During Period, Value, Treasury Stock Reissued	(28,649)			(38,586)	(9,937)
Exercise of stock options and restricted stock units, shares		270,413
Repurchases of common stock	(399,997)			(399,997)
Repurchases of common stock, shares		(749,254)
Adjustment to Additional Paid in Capital, Share-Based Compensation	16,582		16,582
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	1,539				1,539
Net Income (Loss) Attributable to Parent	375,972				375,972
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	89,592					89,592
Ending balance at Dec. 31, 2017	$ 547,280	$ 448	747,138	(3,368,182)	3,433,282	(265,406)
Ending balance, shares at Dec. 31, 2017	25,541,393	25,541,393
Stock Issued During Period, Value, Treasury Stock Reissued	$ (5,669)			(5,900)	(231)
Exercise of stock options and restricted stock units, shares	39,362	39,362
Repurchases of common stock	$ (118,750)			(118,750)
Repurchases of common stock, shares	(187,880)	(187,880)
Adjustment to Additional Paid in Capital, Share-Based Compensation	$ 4,354		4,354
Net Income (Loss) Attributable to Parent	93,304				93,304
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	28,890					28,890
Ending balance at Mar. 31, 2018	$ 560,747	$ 448	$ 751,492	$ (3,481,032)	$ 3,526,355	$ (236,516)
Ending balance, shares at Mar. 31, 2018	25,392,875	25,392,875