Parent Company (Statement of Cash Flows) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Other	$ 350	$ 100	$ (439)
Net cash provided (used) by operating activities	5,699	3,635	5,525
Investing Activities
Other	(1,340)	(952)	(706)
Net cash provided (used) by investing activities	(15,015)	(7,442)	(16,260)
Financing Activities
Net change in commercial paper			1,394
Preferred stock issuances		519
Acquisition of treasury stock	(2,447)	(2,062)	(2,152)
Preferred stock cash dividends paid	(236)	(209)	(219)
Common stock cash dividends paid	(1,266)	(1,061)	(1,038)
Parent Company
Operating Activities
Net Income (Loss) Attributable to Parent	5,338	3,903	4,106
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Equity in undistributed net earnings of subsidiaries	(1,974)	(971)	(993)
Return on investment in subsidiary	98
Other	194	143	149
Net cash provided (used) by operating activities	3,656	3,075	3,262
Investing Activities
Net change in loans and securities from affiliates	114	2,161	(185)
Net change in restricted deposits with banking subsidiary		(175)	400
Net change in nonrestricted interest earning deposits	(1,116)	(1,607)	3,244
Net change in restricted interest-earning deposits		300	(300)
Other		266	(82)
Net cash provided (used) by investing activities	(1,002)	945	3,077
Financing Activities
Net change in other borrowed funds from affiliates	316	(124)	(100)
Net change in senior debt	1,325	(1,252)	(1,888)
Net change in subordinated debt	(580)	17	(580)
Net change in commercial paper	100
Preferred stock issuances		519
Preferred stock redemptions			(500)
Common and treasury stock issuances	132	151	139
Acquisition of treasury stock	(2,447)	(2,062)	(2,152)
Preferred stock cash dividends paid	(236)	(209)	(219)
Common stock cash dividends paid	(1,264)	(1,060)	(1,039)
Net cash provided (used) by financing activities	(2,654)	(4,020)	(6,339)
Cash held at banking subsidiary at beginning of year	1	1	1
Cash held at banking subsidiary at end of year	$ 1	$ 1	$ 1