Parent Company FInancial Statements Parent Company Financial Statements (Statements of Income) (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]
Other interest income									$ 27	$ 17	$ 14
Other noninterest income									102	102	130
Interest on medium- and long-term debt									72	52	50
Salaries and benefits expense									961	1,009	980
Net occupancy expense									157	159	171
Equipment expense									53	53	57
Restructuring charges									93	0	0
Other noninterest expenses									135	160	173
Benefit for income taxes	$ 62	$ 64	$ 42	$ 25	$ 41	$ 63	$ 64	$ 61	193	229	277
Net Income	164	149	104	60	116	136	135	134	477	521	593
Less income allocated to participating securities	1	1	1	1	1	2	1	2	4	6	7
Net income attributable to common shares	$ 163	$ 148	$ 103	$ 59	$ 115	$ 134	$ 134	$ 132	473	515	586
Parent Company
Condensed Financial Statements, Captions [Line Items]
Dividends from subsidiaries									549	441	384
Other interest income									1	1	1
Intercompany management fees									138	123	118
Other noninterest income									3	1	7
Total income									691	566	510
Interest on medium- and long-term debt									10	14	14
Salaries and benefits expense									114	112	114
Net occupancy expense									5	5	5
Equipment expense									1	1	1
Restructuring charges									33	0	0
Other noninterest expenses									72	70	70
Total expenses									235	202	204
Income before benefit for income taxes and equity in undistributed earnings of subsidiaries									456	364	306
Benefit for income taxes									(28)	(27)	(27)
Income before equity in undistributed earnings of subsidiaries									484	391	333
Equity in undistributed earnings of subsidiaries, principally banks									(7)	130	260
Net Income									477	521	593
Less income allocated to participating securities									4	6	7
Net income attributable to common shares									$ 473	$ 515	$ 586