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, 2017	$ 6,231,421	$ 0	$ 124,218	$ 508,476	$ 1,424,274	$ 4,806,208	$ (631,755)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income (loss)	(310,475)				(484,075)	173,600
Common dividends declared ($0.1600 in 2018 and $0.1725 in 2019)	(18,216)					(18,216)
Acquisition of treasury stock	(109,954)						(109,954)
Stock-based compensation	9,060			465		(1,803)	10,398
Exercise of stock options	13,569					(9,077)	22,646
Stockholders' equity, ending balance at Mar. 31, 2018	5,820,301	0	124,218	508,941	940,199	4,955,608	(708,665)
Stockholders' equity, beginning balance at Dec. 31, 2018	5,415,177	0	121,218	524,414	319,475	5,213,468	(763,398)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income (loss)	735,937				550,592	185,345
Common dividends declared ($0.1600 in 2018 and $0.1725 in 2019)	(18,943)					(18,943)
Acquisition of treasury stock	(110,896)						(110,896)
Stock-based compensation	10,559			(5,885)		(6,817)	23,261
Exercise of stock options	12,089					(7,736)	19,825
Stockholders' equity, ending balance at Mar. 31, 2019	$ 6,043,426	$ 0	$ 121,218	$ 518,529	$ 870,067	$ 5,364,820	$ (831,208)