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)		278,832				145,291	133,541
Common dividends declared ($0.15 in 2017 and $0.16 in 2018)		(17,567)					(17,567)
Acquisition of treasury stock		(113,719)						(113,719)
Stock-based compensation		8,195			1,351		(606)	7,450
Exercise of stock options		22,225					(11,501)	33,726
Stockholders' equity, ending balance at Mar. 31, 2017		4,744,827	0	127,218	491,772	722,865	3,994,665	(591,693)
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)		(310,475)				(484,075)	173,600
Common dividends declared ($0.15 in 2017 and $0.16 in 2018)		(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)

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