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)	1,019,672				757,210	262,462
Common dividends declared ($0.28 in 2016 and $0.30 in 2017)	(33,766)					(33,766)
Acquisition of treasury stock	(202,975)						(202,975)
Stock-based compensation	13,989			7,442		(2,224)	8,771
Exercise of stock options	25,286					(23,175)	48,461
Stockholders' equity, ending balance at Jun. 30, 2016	4,877,758	0	130,218	489,726	989,157	3,817,666	(549,009)
Stockholders' equity, beginning balance at Mar. 31, 2016					535,475
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income (loss)	592,111
Stockholders' equity, ending balance at Jun. 30, 2016	4,877,758	0	130,218	489,726	989,157	3,817,666	(549,009)
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)	680,800				406,986	273,814
Common dividends declared ($0.28 in 2016 and $0.30 in 2017)	(35,005)					(35,005)
Acquisition of treasury stock	(203,756)						(203,756)
Stock-based compensation	16,546			9,702		(606)	7,450
Exercise of stock options	27,970					(16,244)	44,214
Stockholders' equity, ending balance at Jun. 30, 2017	5,053,416	0	127,218	500,123	984,560	4,112,757	(671,242)
Stockholders' equity, beginning balance at Mar. 31, 2017					722,865
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income (loss)	401,968
Stockholders' equity, ending balance at Jun. 30, 2017	$ 5,053,416	$ 0	$ 127,218	$ 500,123	$ 984,560	$ 4,112,757	$ (671,242)