Parent Company FInancial Statements Parent Company Financial Statements (Statements of Cash Flows) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Net income	$ 164	$ 149	$ 104	$ 60	$ 116	$ 136	$ 135	$ 134	$ 477	$ 521	$ 593
Depreciation and amortization									121	118	123
Net periodic defined benefit cost									6	48	40
Share-based compensation expense									34	38	38
Benefit for deferred income taxes									(51)	(71)	130
Excess tax benefits from share-based compensation arrangements									(9)	(3)	(7)
Other, net									(355)	105	(243)
Net cash provided by operating activities									493	862	639
Net change in premises and equipment									(95)	(119)	(70)
Net cash (used in) provided by investing activities									(270)	(3,255)	(3,743)
Maturities and redemptions of medium- and long-term debt									(650)	(606)	(1,406)
Issuances of medium- and long-term debt									2,800	1,016	596
Repurchases of common stock									(310)	(240)	(260)
Cash dividends paid on common stock									(152)	(147)	(137)
Issuances of common stock under employee stock plans									152	22	49
Purchase and retirement of warrants									0	(10)	0
Excess tax benefits from share-based compensation arrangements									9	3	7
Other, net									5	5	1
Net cash (used in) provided by financing activities									848	2,469	2,724
Net increase (decrease) in cash and cash equivalents									1,071	76	(380)
Cash and cash equivalents at beginning of period				6,147				6,071	6,147	6,071	6,451
Cash and cash equivalents at end of period	7,218				6,147				7,218	6,147	6,071
Interest paid									111	94	101
Income taxes recovered									106	88	218
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net income									477	521	593
Undistributed earnings of subsidiaries, principally banks									7	(130)	(260)
Depreciation and amortization									1	1	1
Net periodic defined benefit cost									1	5	4
Share-based compensation expense									14	14	16
Benefit for deferred income taxes									(3)	0	0
Excess tax benefits from share-based compensation arrangements									(9)	(3)	(7)
Other, net									6	5	16
Net cash provided by operating activities									494	413	363
Net change in premises and equipment									0	(1)	2
Net cash (used in) provided by investing activities									0	(1)	2
Maturities and redemptions of medium- and long-term debt									0	(600)	0
Issuances of medium- and long-term debt									0	0	596
Repurchases of common stock									(310)	(240)	(260)
Cash dividends paid on common stock									(152)	(147)	(137)
Issuances of common stock under employee stock plans									152	22	49
Purchase and retirement of warrants									0	(10)	0
Excess tax benefits from share-based compensation arrangements									9	3	7
Other, net									(5)	0	0
Net cash (used in) provided by financing activities									(306)	(972)	255
Net increase (decrease) in cash and cash equivalents									188	(560)	620
Cash and cash equivalents at beginning of period				$ 573				$ 1,133	573	1,133	513
Cash and cash equivalents at end of period	$ 761				$ 573				761	573	1,133
Interest paid									9	16	12
Income taxes recovered									$ (139)	$ (62)	$ (33)