Asset Quality - Aging Analysis of Past Due and Current Loans (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Total loans	$ 100,730	$ 101,185
Accrued interest	225	241
Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	69,830	71,980
Accrued interest	127	140
Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	30,900	29,205
Accrued interest	97	101
Commercial and Industrial | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	50,672	52,907
Commercial mortgage | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	12,965	12,687
Commercial real estate: Construction | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	2,132	1,987
Commercial real estate loans | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	15,097	14,674
Commercial lease financing | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	4,061	4,399
Real estate — residential mortgage | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	12,131	9,298
Home equity loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	9,047	9,360
Consumer direct loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	5,049	4,714
Commercial credit card | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	135	127
Commercial credit card | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	923	989
Consumer indirect loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	3,750	4,844
Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Total loans	99,772	100,073
Financial Asset, Not Past Due | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	69,227	71,299
Financial Asset, Not Past Due | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	30,545	28,774
Financial Asset, Not Past Due | Commercial and Industrial | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	50,188	52,396
Financial Asset, Not Past Due | Commercial mortgage | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	12,865	12,548
Financial Asset, Not Past Due | Commercial real estate: Construction | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	2,131	1,986
Financial Asset, Not Past Due | Commercial real estate loans | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	14,996	14,534
Financial Asset, Not Past Due | Commercial lease financing | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	4,043	4,369
Financial Asset, Not Past Due | Real estate — residential mortgage | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	12,023	9,173
Financial Asset, Not Past Due | Home equity loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	8,861	9,143
Financial Asset, Not Past Due | Consumer direct loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	5,035	4,694
Financial Asset, Not Past Due | Commercial credit card | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	909	972
Financial Asset, Not Past Due | Consumer indirect loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	3,717	4,792
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	142	148
30-59 Days Past Due | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	90	66
30-59 Days Past Due | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	52	82
30-59 Days Past Due | Commercial and Industrial | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	71	36
30-59 Days Past Due | Commercial mortgage | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	12	9
30-59 Days Past Due | Commercial real estate: Construction | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	0	0
30-59 Days Past Due | Commercial real estate loans | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	12	9
30-59 Days Past Due | Commercial lease financing | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	7	21
30-59 Days Past Due | Real estate — residential mortgage | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	7	11
30-59 Days Past Due | Home equity loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	23	34
30-59 Days Past Due | Consumer direct loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	5	7
30-59 Days Past Due | Commercial credit card | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	3	5
30-59 Days Past Due | Consumer indirect loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	14	25
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	48	93
60-89 Days Past Due | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	28	56
60-89 Days Past Due | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	20	37
60-89 Days Past Due | Commercial and Industrial | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	19	50
60-89 Days Past Due | Commercial mortgage | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	7	5
60-89 Days Past Due | Commercial real estate: Construction | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	1	0
60-89 Days Past Due | Commercial real estate loans | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	8	5
60-89 Days Past Due | Commercial lease financing | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	1	1
60-89 Days Past Due | Real estate — residential mortgage | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	2	3
60-89 Days Past Due | Home equity loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	10	20
60-89 Days Past Due | Consumer direct loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	2	4
60-89 Days Past Due | Commercial credit card | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	2	3
60-89 Days Past Due | Consumer indirect loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	4	7
90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	74	86
90 and Greater Days Past Due | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	57	62
90 and Greater Days Past Due | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	17	24
90 and Greater Days Past Due | Commercial and Industrial | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	39	40
90 and Greater Days Past Due | Commercial mortgage | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	15	21
90 and Greater Days Past Due | Commercial real estate: Construction | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	0	1
90 and Greater Days Past Due | Commercial real estate loans | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	15	22
90 and Greater Days Past Due | Commercial lease financing | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	3	0
90 and Greater Days Past Due | Real estate — residential mortgage | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	0	1
90 and Greater Days Past Due | Home equity loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	7	9
90 and Greater Days Past Due | Consumer direct loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	3	4
90 and Greater Days Past Due | Commercial credit card | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	6	7
90 and Greater Days Past Due | Consumer indirect loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	1	3
Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Total loans	958	1,112
Financial Asset, Past Due | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	603	681
Financial Asset, Past Due | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	355	431
Financial Asset, Past Due | Commercial and Industrial | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	484	511
Financial Asset, Past Due | Commercial mortgage | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	100	139
Financial Asset, Past Due | Commercial real estate: Construction | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	1	1
Financial Asset, Past Due | Commercial real estate loans | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	101	140
Financial Asset, Past Due | Commercial lease financing | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	18	30
Financial Asset, Past Due | Real estate — residential mortgage | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	108	125
Financial Asset, Past Due | Home equity loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	186	217
Financial Asset, Past Due | Consumer direct loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	14	20
Financial Asset, Past Due | Commercial credit card | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	14	17
Financial Asset, Past Due | Consumer indirect loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	33	52
Nonperforming financial instruments
Financing Receivable, Past Due [Line Items]
Total loans	694	785
Nonperforming financial instruments | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	428	497
Nonperforming financial instruments | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	266	288
Nonperforming financial instruments | Commercial and Industrial | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	355	385
Nonperforming financial instruments | Commercial mortgage | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	66	104
Nonperforming financial instruments | Commercial real estate: Construction | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	0	0
Nonperforming financial instruments | Commercial real estate loans | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	66	104
Nonperforming financial instruments | Commercial lease financing | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	7	8
Nonperforming financial instruments | Real estate — residential mortgage | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	99	110
Nonperforming financial instruments | Home equity loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	146	154
Nonperforming financial instruments | Consumer direct loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	4	5
Nonperforming financial instruments | Commercial credit card | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	3	2
Nonperforming financial instruments | Consumer indirect loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	$ 14	$ 17