Asset Quality - Breakdown of Nonperforming TDRs by Loans Category (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) SecurityLoan	Dec. 31, 2020 USD ($) SecurityLoan
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	4,417	6,133
Pre-modification Outstanding Recorded Investment	$ 375	$ 478
Post-modification Outstanding Recorded Investment	$ 270	$ 363
Prior-Year accruing
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	3,040	3,743
Pre-modification Outstanding Recorded Investment	$ 163	$ 184
Post-modification Outstanding Recorded Investment	$ 124	$ 134
Prior-Year accruing | Commercial Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	8	3
Pre-modification Outstanding Recorded Investment	$ 49	$ 5
Post-modification Outstanding Recorded Investment	$ 26	$ 0
Prior-Year accruing | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	3,032	3,740
Pre-modification Outstanding Recorded Investment	$ 164	$ 179
Post-modification Outstanding Recorded Investment	$ 124	$ 134
Nonperforming financial instruments
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	1,377	2,390
Pre-modification Outstanding Recorded Investment	$ 212	$ 294
Post-modification Outstanding Recorded Investment	$ 146	$ 229
Nonperforming financial instruments | Commercial Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	38	73
Pre-modification Outstanding Recorded Investment	$ 142	$ 198
Post-modification Outstanding Recorded Investment	$ 82	$ 142
Nonperforming financial instruments | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	1,339	2,317
Pre-modification Outstanding Recorded Investment	$ 70	$ 96
Post-modification Outstanding Recorded Investment	$ 64	$ 87
Commercial and Industrial | Prior-Year accruing | Commercial Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	7	3
Pre-modification Outstanding Recorded Investment	$ 49	$ 5
Post-modification Outstanding Recorded Investment	$ 26	$ 0
Commercial and Industrial | Nonperforming financial instruments | Commercial Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	35	66
Pre-modification Outstanding Recorded Investment	$ 92	$ 136
Post-modification Outstanding Recorded Investment	$ 53	$ 92
Commercial mortgage | Prior-Year accruing | Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	1	0
Pre-modification Outstanding Recorded Investment	$ 0	$ 0
Post-modification Outstanding Recorded Investment	$ 0	$ 0
Commercial mortgage | Nonperforming financial instruments | Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	3	7
Pre-modification Outstanding Recorded Investment	$ 50	$ 62
Post-modification Outstanding Recorded Investment	$ 29	$ 50
Commercial real estate loans | Prior-Year accruing | Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	1	0
Pre-modification Outstanding Recorded Investment	$ 0	$ 0
Post-modification Outstanding Recorded Investment	$ 0	$ 0
Commercial real estate loans | Nonperforming financial instruments | Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	3	7
Pre-modification Outstanding Recorded Investment	$ 50	$ 62
Post-modification Outstanding Recorded Investment	$ 29	$ 50
Real estate — residential mortgage | Prior-Year accruing | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	464	485
Pre-modification Outstanding Recorded Investment	$ 39	$ 37
Post-modification Outstanding Recorded Investment	$ 34	$ 31
Real estate — residential mortgage | Nonperforming financial instruments | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	218	258
Pre-modification Outstanding Recorded Investment	$ 26	$ 35
Post-modification Outstanding Recorded Investment	$ 25	$ 34
Home equity loans | Prior-Year accruing | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	1,648	1,781
Pre-modification Outstanding Recorded Investment	$ 100	$ 106
Post-modification Outstanding Recorded Investment	$ 78	$ 83
Home equity loans | Nonperforming financial instruments | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	569	630
Pre-modification Outstanding Recorded Investment	$ 38	$ 41
Post-modification Outstanding Recorded Investment	$ 33	$ 37
Consumer direct loans | Prior-Year accruing | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	206	163
Pre-modification Outstanding Recorded Investment	$ 5	$ 4
Post-modification Outstanding Recorded Investment	$ 3	$ 3
Consumer direct loans | Nonperforming financial instruments | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	201	212
Pre-modification Outstanding Recorded Investment	$ 3	$ 3
Post-modification Outstanding Recorded Investment	$ 3	$ 3
Credit cards | Prior-Year accruing | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	564	536
Pre-modification Outstanding Recorded Investment	$ 4	$ 3
Post-modification Outstanding Recorded Investment	$ 1	$ 1
Credit cards | Nonperforming financial instruments | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	328	356
Pre-modification Outstanding Recorded Investment	$ 2	$ 2
Post-modification Outstanding Recorded Investment	$ 2	$ 2
Consumer indirect loans | Prior-Year accruing | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	150	775
Pre-modification Outstanding Recorded Investment	$ 16	$ 29
Post-modification Outstanding Recorded Investment	$ 8	$ 16
Consumer indirect loans | Nonperforming financial instruments | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	23	861
Pre-modification Outstanding Recorded Investment	$ 1	$ 15
Post-modification Outstanding Recorded Investment	$ 1	$ 11