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
Treasury Stock, Value, Acquired, Cost Method	399,997			399,997
Repurchases of common stock, shares		(749,254)
Tax benefit resulting from exercise of certain employee stock options	1,539
Adjustment to Additional Paid in Capital, Share-Based Compensation	16,582		16,582
Net earnings	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
Cumulative Effect of New Accounting Principle in Period of Adoption					1,539
Stock Issued During Period, Value, Treasury Stock Reissued	$ 9,960			10,385	425
Exercise of stock options and restricted stock units, shares	68,653	68,653
Treasury Stock, Value, Acquired, Cost Method	$ 237,499			237,499
Repurchases of common stock, shares	(396,218)	(396,218)
Adjustment to Additional Paid in Capital, Share-Based Compensation	$ 8,236		8,236
Net earnings	204,772				204,772
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(315)					(315)
Ending balance at Jun. 30, 2018	$ 532,434	$ 448	$ 755,374	$ (3,595,296)	$ 3,637,629	$ (265,721)
Ending balance, shares at Jun. 30, 2018	25,213,828	25,213,828