Condensed Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands		Total	Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Treasury Stock
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
Comprehensive income (loss)		(310,475)				(484,075)	173,600
Common dividends declared		(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
Ending balance at Mar. 31, 2018		5,820,301	0	124,218	508,941	940,199	4,955,608	(708,665)
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
Comprehensive income (loss)		(462,750)
Ending balance at Jun. 30, 2018		5,571,609	0	124,218	517,077	603,499	5,115,071	(788,256)
Beginning balance at Mar. 31, 2018		5,820,301	0	124,218	508,941	940,199	4,955,608	(708,665)
Increase (Decrease) in Stockholders' Equity
Comprehensive income (loss)		(152,275)				(336,700)	184,425
Common dividends declared		(18,062)					(18,062)
Acquisition of treasury stock		(96,635)						(96,635)
Stock-based compensation		10,497			8,136		0	2,361
Exercise of stock options		7,783					(6,900)	14,683
Ending balance at Jun. 30, 2018		5,571,609	0	124,218	517,077	603,499	5,115,071	(788,256)
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
Comprehensive income (loss)		735,937				550,592	185,345
Common dividends declared		(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
Ending balance at Mar. 31, 2019		6,043,426	0	121,218	518,529	870,067	5,364,820	(831,208)
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
Comprehensive income (loss)		1,491,686
Ending balance at Jun. 30, 2019		6,700,398	0	121,218	526,518	1,439,250	5,514,517	(901,105)
Beginning balance at Mar. 31, 2019		6,043,426	0	121,218	518,529	870,067	5,364,820	(831,208)
Increase (Decrease) in Stockholders' Equity
Comprehensive income (loss)		755,749				569,183	186,566
Common dividends declared		(18,838)					(18,838)
Acquisition of treasury stock		(119,201)						(119,201)
Stock-based compensation		11,256			7,989		0	3,267
Exercise of stock options		27,901					(18,136)	46,037
Ending balance at Jun. 30, 2019		6,700,398	$ 0	$ 121,218	$ 526,518	$ 1,439,250	5,514,517	$ (901,105)
Increase (Decrease) in Stockholders' Equity
Adoption of ASU 2016-01	[1]	$ 105					$ 105

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