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)		680,800				406,986	273,814
Common dividends declared ($0.30 in 2017 and $0.32 in 2018)		(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)
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)		(462,750)				(820,775)	358,025
Common dividends declared ($0.30 in 2017 and $0.32 in 2018)		(36,278)					(36,278)
Acquisition of treasury stock		(206,589)						(206,589)
Stock-based compensation		19,557			8,601		(1,803)	12,759
Exercise of stock options		21,352					(15,977)	37,329
Stockholders' equity, ending balance at Jun. 30, 2018		5,571,609	0	124,218	517,077	603,499	5,115,071	(788,256)
Stockholders' equity, beginning balance at Mar. 31, 2018						940,199
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income (loss)		(152,275)
Stockholders' equity, ending balance at Jun. 30, 2018		$ 5,571,609	$ 0	$ 124,218	$ 517,077	$ 603,499	$ 5,115,071	$ (788,256)

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