Condensed Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Treasury Stock [Member]
Stockholders' equity, beginning balance at Dec. 31, 2015	$ 4,055,552	$ 0	$ 130,218	$ 482,284	$ 231,947	$ 3,614,369	$ (403,266)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income (loss)	1,325,210				910,879	414,331
Common dividends declared ($0.42 in 2016 and $0.45 in 2017)	(50,410)					(50,410)
Acquisition of treasury stock	(311,356)						(311,356)
Stock-based compensation	20,334			13,787		(2,224)	8,771
Exercise of stock options	47,053					(42,040)	89,093
Stockholders' equity, ending balance at Sep. 30, 2016	5,086,383	0	130,218	496,071	1,142,826	3,934,026	(616,758)
Stockholders' equity, beginning balance at Jun. 30, 2016					989,157
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income (loss)	305,538
Stockholders' equity, ending balance at Sep. 30, 2016	5,086,383	0	130,218	496,071	1,142,826	3,934,026	(616,758)
Stockholders' equity, beginning balance at Dec. 31, 2016	4,566,861	0	127,218	490,421	577,574	3,890,798	(519,150)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income (loss)	888,876				461,728	427,148
Common dividends declared ($0.42 in 2016 and $0.45 in 2017)	(52,304)					(52,304)
Acquisition of treasury stock	(301,448)						(301,448)
Stock-based compensation	24,809			17,965		(606)	7,450
Exercise of stock options	40,891					(25,454)	66,345
Stockholders' equity, ending balance at Sep. 30, 2017	5,167,685	0	127,218	508,386	1,039,302	4,239,582	(746,803)
Stockholders' equity, beginning balance at Jun. 30, 2017					984,560
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income (loss)	208,076
Stockholders' equity, ending balance at Sep. 30, 2017	$ 5,167,685	$ 0	$ 127,218	$ 508,386	$ 1,039,302	$ 4,239,582	$ (746,803)