Parent Company FInancial Statements Parent Company Financial Statements (Statements of Cash Flows) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Net income	$ 116	$ 136	$ 135	$ 134	$ 149	$ 154	$ 151	$ 139	$ 521	$ 593	$ 541
Depreciation and amortization									118	123	122
Net periodic defined benefit cost									48	40	88
Share-based compensation expense									38	38	35
Provision for deferred income taxes									(71)	130	(20)
Excess tax benefits from share-based compensation arrangements									(3)	(7)	(3)
Other, net									105	(243)	(2)
Net cash provided by operating activities									862	639	836
Net change in premises and equipment									(119)	(70)	(102)
Net cash (used in) provided by investing activities									(3,255)	(3,743)	1,174
Maturities and redemptions of medium- and long-term debt									(606)	(1,406)	(1,080)
Issuances of medium- and long-term debt									1,016	596	0
Repurchases of common stock									(240)	(260)	(291)
Cash dividends paid on common stock									(147)	(137)	(123)
Issuances of common stock under employee stock plans									22	49	33
Purchase and retirement of warrants									(10)	0	0
Excess tax benefits from share-based compensation arrangements									3	7	3
Net cash (used in) provided by financing activities									2,469	2,724	(93)
Net (decrease) increase in cash and cash equivalents									76	(380)	1,917
Cash and cash equivalents at beginning of period				6,071				6,451	6,071	6,451	4,534
Cash and cash equivalents at end of period	6,147				6,071				6,147	6,071	6,451
Interest paid									94	101	114
Income taxes recovered									88	218	115
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net income									521	593	541
Undistributed earnings of subsidiaries, principally banks									(130)	(260)	(108)
Depreciation and amortization									1	1	1
Net periodic defined benefit cost									5	4	8
Share-based compensation expense									14	16	14
Provision for deferred income taxes									0	0	3
Excess tax benefits from share-based compensation arrangements									(3)	(7)	(3)
Other, net									5	16	2
Net cash provided by operating activities									413	363	458
Net change in premises and equipment									(1)	2	0
Net cash (used in) provided by investing activities									(1)	2	0
Maturities and redemptions of medium- and long-term debt									(600)	0	0
Issuances of medium- and long-term debt									0	596	0
Repurchases of common stock									(240)	(260)	(291)
Cash dividends paid on common stock									(147)	(137)	(123)
Issuances of common stock under employee stock plans									22	49	33
Purchase and retirement of warrants									(10)	0	0
Excess tax benefits from share-based compensation arrangements									3	7	3
Net cash (used in) provided by financing activities									(972)	255	(378)
Net (decrease) increase in cash and cash equivalents									(560)	620	80
Cash and cash equivalents at beginning of period				$ 1,133				$ 513	1,133	513	433
Cash and cash equivalents at end of period	$ 573				$ 1,133				573	1,133	513
Interest paid									16	12	11
Income taxes recovered									$ (62)	$ (33)	$ (27)