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, 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 ($0.45 in 2017 and $0.48 in 2018)		(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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of ASU 2016-01	[1]	4,896					4,896
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)		(409,901)				(946,650)	536,749
Common dividends declared ($0.45 in 2017 and $0.48 in 2018)		(54,231)					(54,231)
Acquisition of treasury stock		(298,314)						(298,314)
Stock-based compensation		29,969			19,013		(1,803)	12,759
Exercise of stock options		35,065					(23,855)	58,920
Stockholders' equity, ending balance at Sep. 30, 2018		5,538,905	0	124,218	527,489	477,624	5,267,964	(858,390)
Stockholders' equity, beginning balance at Jun. 30, 2018						603,499
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income (loss)		52,849
Stockholders' equity, ending balance at Sep. 30, 2018		$ 5,538,905	$ 0	$ 124,218	$ 527,489	$ 477,624	$ 5,267,964	$ (858,390)

[1] See further discussion in Note 2—New Accounting Standards and Note 4—Investments.