Consolidated Statements of Shareholder's 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, 2014		$ 4,697,466	$ 0	$ 134,218	$ 457,613	$ 997,452	$ 3,376,846	$ (268,663)
Increase (Decrease) in Stockholders' Equity
Comprehensive income (loss)		(238,405)				(765,505)	527,100
Common dividends declared ($.54 in 2015, $.56 in 2016 and $.60 in 2017 a share)		(67,182)					(67,182)
Acquisition of treasury stock		(418,526)						(418,526)
Stock-based compensation		28,664			21,813		(2,132)	8,983
Exercise of stock options		53,535			17,577		(36,322)	72,280
Retirement of treasury stock		0		(4,000)	(14,719)		(183,941)	202,660
Ending balance at Dec. 31, 2015		4,055,552	0	130,218	482,284	231,947	3,614,369	(403,266)
Increase (Decrease) in Stockholders' Equity
Comprehensive income (loss)		895,406				345,627	549,779
Common dividends declared ($.54 in 2015, $.56 in 2016 and $.60 in 2017 a share)		(66,968)					(66,968)
Acquisition of treasury stock		(404,784)						(404,784)
Stock-based compensation		26,326			19,659		(2,224)	8,891
Exercise of stock options		61,329					(53,845)	115,174
Retirement of treasury stock		0		(3,000)	(11,522)		(150,313)	164,835
Ending balance at Dec. 31, 2016		4,566,861	0	127,218	490,421	577,574	3,890,798	(519,150)
Increase (Decrease) in Stockholders' Equity
Comprehensive income (loss)		2,048,794				594,300	1,454,494
Common dividends declared ($.54 in 2015, $.56 in 2016 and $.60 in 2017 a share)		(69,494)					(69,494)
Acquisition of treasury stock		(412,989)						(412,989)
Stock-based compensation		37,034			30,190		(606)	7,450
Exercise of stock options		61,215					(38,333)	99,548
Reclassification, Tax Reform	[1]					252,400	(252,400)
Retirement of treasury stock		0		(3,000)	(12,135)		(178,251)	193,386
Ending balance at Dec. 31, 2017		$ 6,231,421	$ 0	$ 124,218	$ 508,476	$ 1,424,274	$ 4,806,208	$ (631,755)

[1] Income tax effects of certain items were reclassified from accumulated other comprehensive income to retained earnings to remove stranded tax effects as a result of early adoption of ASU 2018-02, Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. See further discussion in Note 1—Significant Accounting Policies.