Consolidated Statements of Changes in Equity - USD ($) $ in Millions		Total	Series D Preferred Stock [Member]	Series E Preferred Stock [Member]	Preferred Stock [Member]	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Retained Earnings [Member] Series A Preferred Stock [Member]	Retained Earnings [Member] Series C Preferred Stock [Member]	Retained Earnings [Member] Series D Preferred Stock [Member]	Retained Earnings [Member] Series E Preferred Stock [Member]	Treasury Stock, at Cost [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Noncontrolling Interests [Member]
Beginning Balance, Preferred Shares (in shares) at Dec. 31, 2016					17,421,000
Beginning Balance, Common Shares (in shares) at Dec. 31, 2016						1,079,314,000
Beginning Balance at Dec. 31, 2016					$ 1,665	$ 1,257	$ 6,385	$ 9,378					$ (2,904)	$ (541)	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)		$ 1,101						1,100							1
Other comprehensive income (loss)		35												35
Deferred compensation							11
Cash dividends declared on common shares								(296)
Cash dividends declared on preferred stock									$ (6)	$ (7)	$ (20)	$ (24)
Open market common share repurchases (in shares)						(25,564,000)
Open market Common Share repurchases													(466)
Employee equity compensation program common share repurchase (in shares)						(3,479,000)
Employee equity compensation program Common Share repurchases													(65)
Series A Preferred Stock exchanged for common shares (in shares)					2,900,000	20,568,000
Series A Preferred Stock exchanged for common shares					$ (290)		(49)						338
Redemption of Series C Preferred Stock (in shares)					(14,000,000)
Redemption of Series C Preferred Stock		350			$ (350)
Common shares reissued (returned) for stock options and other employee benefit plans, shares						8,200,000
Series A Preferred Stock exchanged for Common Shares							(37)						135
Ending Balance, Preferred Shares (in shares) at Sep. 30, 2017					521,000
Ending Balance, Common Shares (in shares) at Sep. 30, 2017						1,079,039,000
Ending Balance at Sep. 30, 2017					$ 1,025	$ 1,257	6,310	10,125					(2,962)	(506)	1
Beginning Balance, Preferred Shares (in shares) at Jun. 30, 2017					521,000
Beginning Balance, Common Shares (in shares) at Jun. 30, 2017						1,092,739,000
Beginning Balance at Jun. 30, 2017					$ 1,025	$ 1,257	6,310	9,878					(2,711)	(506)	2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)		364						364							0
Other comprehensive income (loss)		0												0
Deferred compensation							4
Cash dividends declared on common shares								(102)
Cash dividends declared on preferred stock											(7)	(8)
Open market common share repurchases (in shares)						(14,962,000)
Open market Common Share repurchases													(270)
Employee equity compensation program common share repurchase (in shares)						(336,000)
Employee equity compensation program Common Share repurchases													(6)
Common shares reissued (returned) for stock options and other employee benefit plans, shares						1,598,000
Series A Preferred Stock exchanged for Common Shares							(4)						25
Net contribution from (distribution to) noncontrolling interests															(1)
Ending Balance, Preferred Shares (in shares) at Sep. 30, 2017					521,000
Ending Balance, Common Shares (in shares) at Sep. 30, 2017						1,079,039,000
Ending Balance at Sep. 30, 2017					$ 1,025	$ 1,257	6,310	10,125					(2,962)	(506)	1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect from changes in accounting principle	[1]							(2)
Beginning Balance, Preferred Shares (in shares) at Dec. 31, 2017					521,000
Beginning Balance, Common Shares (in shares) at Dec. 31, 2017						1,069,084,000
Beginning Balance at Dec. 31, 2017		15,025			$ 1,025	$ 1,257	6,335	10,335					(3,150)	(779)	2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other reclassification of AOCI		(5)						5
Net income (loss)		1,382						1,382							0
Other comprehensive income (loss)		(387)												(387)
Deferred compensation							16
Cash dividends declared on common shares								(415)
Cash dividends declared on preferred stock											(20)	(23)
Issuance of Series F Preferred Stock (in shares)					425,000
Issuance of Series F Preferred Stock					$ 425		(13)
Open market common share repurchases (in shares)						(38,806,000)
Open market Common Share repurchases													(820)
Employee equity compensation program common share repurchase (in shares)						(2,271,000)
Employee equity compensation program Common Share repurchases													(47)
Redemption of Series C Preferred Stock		0
Common shares reissued (returned) for stock options and other employee benefit plans, shares						6,280,000
Series A Preferred Stock exchanged for Common Shares							(23)						107
Ending Balance, Preferred Shares (in shares) at Sep. 30, 2018			21,000	500,000	946,000
Ending Balance, Common Shares (in shares) at Sep. 30, 2018						1,034,287,000
Ending Balance at Sep. 30, 2018		15,210			$ 1,450	$ 1,257	6,315	11,262					(3,910)	(1,166)	2
Beginning Balance, Preferred Shares (in shares) at Jun. 30, 2018					521,000
Beginning Balance, Common Shares (in shares) at Jun. 30, 2018						1,058,944,000
Beginning Balance at Jun. 30, 2018					$ 1,025	$ 1,257	6,315	10,970					(3,382)	(1,085)	2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other reclassification of AOCI		0
Net income (loss)		482						482							0
Other comprehensive income (loss)		(81)												(81)
Deferred compensation							4
Cash dividends declared on common shares								(176)
Cash dividends declared on preferred stock											$ (7)	$ (7)
Issuance of Series F Preferred Stock (in shares)					425,000
Issuance of Series F Preferred Stock					$ 425		(13)
Open market common share repurchases (in shares)						(25,368,000)
Open market Common Share repurchases													(541)
Employee equity compensation program common share repurchase (in shares)						(50,000)
Employee equity compensation program Common Share repurchases													(1)
Common shares reissued (returned) for stock options and other employee benefit plans, shares						761,000
Series A Preferred Stock exchanged for Common Shares							9						14
Net contribution from (distribution to) noncontrolling interests															0
Ending Balance, Preferred Shares (in shares) at Sep. 30, 2018			21,000	500,000	946,000
Ending Balance, Common Shares (in shares) at Sep. 30, 2018						1,034,287,000
Ending Balance at Sep. 30, 2018		$ 15,210			$ 1,450	$ 1,257	$ 6,315	$ 11,262					$ (3,910)	$ (1,166)	$ 2

[1] Includes the impact of implementing ASU 2014-09, ASU 2016-01, and ASU 2017-12.