Asset Quality - Aging Analysis of Past Due and Current Loans (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Total loans	$ 98,609	$ 101,185
Accrued interest	211	241
Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	69,329	71,980
Accrued interest	117	140
Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	29,280	29,205
Accrued interest	95	101
Commercial and Industrial | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	49,553	52,907
Commercial mortgage | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	13,674	12,687
Commercial real estate: Construction | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	2,120	1,987
Commercial real estate loans | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	15,794	14,674
Commercial lease financing | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	3,982	4,399
Real estate — residential mortgage | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	14,204	9,298
Home equity loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	8,747	9,360
Consumer direct loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	5,324	4,714
Commercial credit card | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	139	127
Commercial credit card | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	928	989
Consumer indirect loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	77	4,844
Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Total loans	97,809	100,073
Financial Asset, Not Past Due | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	68,858	71,299
Financial Asset, Not Past Due | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	28,951	28,774
Financial Asset, Not Past Due | Commercial and Industrial | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	49,161	52,396
Financial Asset, Not Past Due | Commercial mortgage | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	13,606	12,548
Financial Asset, Not Past Due | Commercial real estate: Construction | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	2,118	1,986
Financial Asset, Not Past Due | Commercial real estate loans | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	15,724	14,534
Financial Asset, Not Past Due | Commercial lease financing | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	3,973	4,369
Financial Asset, Not Past Due | Real estate — residential mortgage | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	14,096	9,173
Financial Asset, Not Past Due | Home equity loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	8,559	9,143
Financial Asset, Not Past Due | Consumer direct loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	5,307	4,694
Financial Asset, Not Past Due | Commercial credit card | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	914	972
Financial Asset, Not Past Due | Consumer indirect loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	75	4,792
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	109	148
30-59 Days Past Due | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	71	66
30-59 Days Past Due | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	38	82
30-59 Days Past Due | Commercial and Industrial | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	56	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	1	0
30-59 Days Past Due | Commercial real estate loans | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	13	9
30-59 Days Past Due | Commercial lease financing | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	2	21
30-59 Days Past Due | Real estate — residential mortgage | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	6	11
30-59 Days Past Due | Home equity loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	22	34
30-59 Days Past Due | Consumer direct loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	6	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	1	25
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	55	93
60-89 Days Past Due | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	36	56
60-89 Days Past Due | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	19	37
60-89 Days Past Due | Commercial and Industrial | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	33	50
60-89 Days Past Due | Commercial mortgage | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	2	5
60-89 Days Past Due | Commercial real estate: Construction | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	0	0
60-89 Days Past Due | Commercial real estate loans | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	2	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	3	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	3	4
60-89 Days Past Due | Commercial credit card | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	3	3
60-89 Days Past Due | Consumer indirect loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	0	7
90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	82	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	25	24
90 and Greater Days Past Due | Commercial and Industrial | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	50	40
90 and Greater Days Past Due | Commercial mortgage | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	5	21
90 and Greater Days Past Due | Commercial real estate: Construction | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	1	1
90 and Greater Days Past Due | Commercial real estate loans | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	6	22
90 and Greater Days Past Due | Commercial lease financing | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	1	0
90 and Greater Days Past Due | Real estate — residential mortgage | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	6	1
90 and Greater Days Past Due | Home equity loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	10	9
90 and Greater Days Past Due | Consumer direct loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	4	4
90 and Greater Days Past Due | Commercial credit card | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	5	7
90 and Greater Days Past Due | Consumer indirect loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	0	3
Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Total loans	800	1,112
Financial Asset, Past Due | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	471	681
Financial Asset, Past Due | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	329	431
Financial Asset, Past Due | Commercial and Industrial | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	392	511
Financial Asset, Past Due | Commercial mortgage | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	68	139
Financial Asset, Past Due | Commercial real estate: Construction | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	2	1
Financial Asset, Past Due | Commercial real estate loans | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	70	140
Financial Asset, Past Due | Commercial lease financing | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	9	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	188	217
Financial Asset, Past Due | Consumer direct loans | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	17	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	2	52
Nonperforming financial instruments
Financing Receivable, Past Due [Line Items]
Total loans	554	785
Nonperforming financial instruments | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	307	497
Nonperforming financial instruments | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	247	288
Nonperforming financial instruments | Commercial and Industrial | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	253	385
Nonperforming financial instruments | Commercial mortgage | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	49	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	49	104
Nonperforming financial instruments | Commercial lease financing | Commercial Loans
Financing Receivable, Past Due [Line Items]
Total loans	5	8
Nonperforming financial instruments | Real estate — residential mortgage | Consumer Loans
Financing Receivable, Past Due [Line Items]
Total loans	93	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	$ 1	$ 17