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)	(409,901)
Ending balance at Sep. 30, 2018	5,538,905	0	124,218	527,489	477,624	5,267,964	(858,390)
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)
Increase (Decrease) in Stockholders' Equity
Comprehensive income (loss)	52,849				(125,875)	178,724
Common dividends declared	(17,953)					(17,953)
Acquisition of treasury stock	(91,725)						(91,725)
Stock-based compensation	10,412			10,412		0	0
Exercise of stock options	13,713					(7,878)	21,591
Ending balance at Sep. 30, 2018	5,538,905	0	124,218	527,489	477,624	5,267,964	(858,390)
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)	2,197,378
Ending balance at Sep. 30, 2019	7,312,531	0	121,218	538,051	1,943,124	5,686,636	(976,498)
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
Other	105					105
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
Comprehensive income (loss)	705,692				503,874	201,818
Common dividends declared	(18,695)					(18,695)
Acquisition of treasury stock	(104,372)						(104,372)
Stock-based compensation	11,533			11,533		0	0
Exercise of stock options	17,975					(11,004)	28,979
Ending balance at Sep. 30, 2019	$ 7,312,531	$ 0	$ 121,218	$ 538,051	$ 1,943,124	$ 5,686,636	$ (976,498)