Consolidated Statement of Changes in Equity - USD ($) $ in Millions	Total	Cumulative effect, period of adoption, adjustment	Total	Total Cumulative effect, period of adoption, adjustment	Common Shares	Common Stock and Capital Surplus	Retained Earnings	Retained Earnings Cumulative effect, period of adoption, adjustment	Accumulated Other Comprehensive Inc. (Loss)	Accumulated Other Comprehensive Inc. (Loss) Cumulative effect, period of adoption, adjustment	Noncontrolling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of accounting change	$ 5,331	$ 4	$ 5,330	$ 4		$ 1,269	$ 3,248	$ 225	$ 813	$ (221)	$ 1
Beginning Balance, shares at Dec. 31, 2017					88,275,460
Beginning Balance at Dec. 31, 2017	5,331	4	5,330	4		1,269	3,248	225	813	(221)	1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of accounting change	4,972	$ 4	4,970	$ 4		1,334	3,588	225	48	(221)	2
Net income (loss) attributable to parent	530		530				530
Net income (loss), including portion attributable to nonredeemable noncontrolling interest	531										1
Other Comprehensive Income (Loss), Net of Tax, including Portion Attributable to Noncontrolling Interest	(544)
Other comprehensive income (loss)	(544)		(544)						(544)		0
Dividends	(397)		(397)				(397)
Shares issued:
Exercise of stock options, shares					778,270
Exercise of stock options	29		29			29
Restricted stock awards, shares					200,625
Restricted stock awards	0		0			0
Other benefit plans, shares					103,797
Other benefit plans	12		12			12
Dividend reinvestment plan, shares					29,998
Dividend reinvestment plan	3		3			3
Stock-based compensation: Expense	23		23			23
Shares acquired and retired, shares					(65,589)
Shares acquired and retired	(6)		(6)			(1)	(5)
Shares exchanged — benefit plans, shares					(26,520)
Shares exchanged — benefit plans	(3)		(3)			(1)	(2)
Forfeitures of restricted stock, shares					(4,317)
Forfeitures of restricted stock	0		0			0
Other	(11)		(11)				(11)				0
Ending Balance, shares at Dec. 31, 2018					89,291,724
Ending Balance at Dec. 31, 2018	4,972		4,970			1,334	3,588		48		2
Beginning Balance, Redeemable Noncontrolling Interests at Dec. 31, 2017	3
Redeemable Noncontrolling Interest, Equity, Carrying Amount [Abstract]
Net income (loss) attributable to redeemable noncontrolling interest	(14)
Other	11
Ending Balance, Redeemable Noncontrolling Interests at Dec. 31, 2018	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of accounting change	4,972		4,970			1,334	3,588		48		2
Cumulative effect of accounting change	4,972		6,269			1,397	4,009	7	863	0	0
Net income (loss) attributable to parent	897		897				897
Net income (loss), including portion attributable to nonredeemable noncontrolling interest	895										(2)
Other Comprehensive Income (Loss), Net of Tax, including Portion Attributable to Noncontrolling Interest	815
Other comprehensive income (loss)	815		815						815		0
Dividends	(446)		(446)				(446)
Shares issued:
Exercise of stock options, shares					747,167
Exercise of stock options	31		31			31
Restricted stock awards, shares					232,635
Restricted stock awards	0		0			0
Other benefit plans, shares					77,429
Other benefit plans	8		8			8
Dividend reinvestment plan, shares					19,334
Dividend reinvestment plan	2		2			2
Stock-based compensation: Expense	23		23			23
Shares exchanged — benefit plans, shares					(50,062)
Shares exchanged — benefit plans	(5)		(5)			(1)	(4)
Forfeitures of restricted stock, shares					(14,541)
Forfeitures of restricted stock	0		0			0
Other	$ (26)		(26)				(26)		0		0
Ending Balance, shares at Dec. 31, 2019	90,303,686				90,303,686
Ending Balance at Dec. 31, 2019	$ 6,269		6,269			1,397	4,009	7	863	0	0
Redeemable Noncontrolling Interest, Equity, Carrying Amount [Abstract]
Net income (loss) attributable to redeemable noncontrolling interest	(26)
Other	26
Ending Balance, Redeemable Noncontrolling Interests at Dec. 31, 2019	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of accounting change	6,269		6,269			1,397	4,009	7	863	0	0
Cumulative effect of accounting change	6,269		6,789			1,367	4,149	$ 7	1,273	$ 0	0
Net income (loss) attributable to parent	732		732				732
Net income (loss), including portion attributable to nonredeemable noncontrolling interest	734										2
Other Comprehensive Income (Loss), Net of Tax, including Portion Attributable to Noncontrolling Interest	414
Other comprehensive income (loss)	416		414						414		0
Dividends	$ (336)		(336)				(336)
Shares issued:
Exercise of stock options, shares	328,471				328,471
Exercise of stock options	$ 14		14			14
Restricted stock awards, shares					227,867
Restricted stock awards	0		0			0
Other benefit plans, shares					143,270
Other benefit plans	10		10			10
Dividend reinvestment plan, shares					18,690
Dividend reinvestment plan	2		2			2
Stock-based compensation: Expense	20		20			20
Shares acquired and retired, shares					(4,531,394)
Shares acquired and retired	(313)		(313)			(70)	(243)
Shares exchanged — benefit plans, shares					(101,663)
Shares exchanged — benefit plans	(11)		(11)			(2)	(9)
Forfeitures of restricted stock, shares					(43,681)
Forfeitures of restricted stock	0		0			0
Other	$ (21)		(19)			(4)	(11)		(4)		(2)
Ending Balance, shares at Dec. 31, 2020	86,345,246				86,345,246
Ending Balance at Dec. 31, 2020	$ 6,789		6,789			1,367	4,149		1,273		0
Redeemable Noncontrolling Interest, Equity, Carrying Amount [Abstract]
Net income (loss) attributable to redeemable noncontrolling interest	(13)
Other comprehensive income (loss), net of tax attributable to redeemable noncontrolling interests	2
Other	11
Ending Balance, Redeemable Noncontrolling Interests at Dec. 31, 2020	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of accounting change	$ 6,789		$ 6,789			$ 1,367	$ 4,149		$ 1,273		$ 0