CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock Member	Additional Paid In Capital Member	Retained Earnings Member	Treasury Stock Member	Accumulated Other Comprehensive Income Member
Total stockholders equity at Dec. 31, 2014	$ 7,023,452	$ 791	$ 1,798,279	$ 4,239,647	$ (672,394)	$ 1,657,129
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss) Attributable to Parent	502,166			502,166
Other Comprehensive Income (Loss), Net of Tax	(948,845)					(948,845)
Dividends to stockholders	(93,381)			(93,381)
Purchase of treasury stock	(384,519)				(384,519)
Reissuance of treasury stock	(36,508)		(17,863)	(28,129)	(46,774)
Total stockholders equity at Dec. 31, 2015	6,135,381	791	1,816,142	4,620,303	(1,010,139)	708,284
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss) Attributable to Parent	701,443			701,443
Other Comprehensive Income (Loss), Net of Tax	431,045					431,045
Dividends to stockholders	(100,371)			(100,371)
Purchase of treasury stock	(122,916)				(122,916)
Reissuance of treasury stock	(48,500)		(32,469)	(22,245)	(38,276)
Total stockholders equity at Dec. 31, 2016	7,093,082	791	1,848,611	5,199,130	(1,094,779)	1,139,329
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | Accounting Standards Update 2018 02 [Member]	17,723			(138,649)		156,372
Net Income (Loss) Attributable to Parent	1,822,181			1,822,181
Other Comprehensive Income (Loss), Net of Tax	767,930					767,930
Dividends to stockholders	(117,291)			(117,291)
Purchase of treasury stock	(43,508)				(43,508)
Reissuance of treasury stock	(29,418)		(22,295)	(29,106)	(36,229)
Total stockholders equity at Dec. 31, 2017	$ 9,569,535	$ 791	$ 1,870,906	$ 6,736,265	$ (1,102,058)	$ 2,063,631