CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands	Total	Common Stock Member	Additional Paid In Capital Member	Retained Earnings Member	Accumulated Other Comprehensive Income (Loss) Member	Treasury Stock Member
Beginning Balance at Dec. 31, 2016		$ 49,187	$ 129,483	$ 1,754,907	$ (291,756)	$ (350,630)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings				214,891
Other comprehensive income, net of tax	$ 74,916				74,916
Dividends paid/declared				(24,740)
Restricted stock			(12,104)			12,105
Stock options exercised			(5,724)			19,902
Other		0	(2,237)	(734)		889
Share-based compensation			11,191			381
Repurchases of common stock						(52,127)
Ending Balance at Dec. 31, 2017	1,527,800	49,187	120,609	1,944,324	(216,840)	(369,480)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2014-09 [Member]				(2,274)
Net earnings	43,643			43,643
Net earnings | Accounting Standards Update 2014-09 [Member]	(1,787)
Other comprehensive income, net of tax	18,033				18,033
Dividends paid/declared				(6,642)
Restricted stock			(6,828)			6,828
Stock options exercised			(1,237)			7,389
Other			(725)			725
Share-based compensation			4,402			189
Repurchases of common stock						(12,328)
Ending Balance at Mar. 31, 2018	$ 1,578,975	$ 49,187	$ 116,221	$ 1,979,051	$ (198,807)	$ (366,677)