Asset Quality - Breakdown of Nonperforming TDRs by Loans Category (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) SecurityLoan	Dec. 31, 2019 USD ($) SecurityLoan
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	6,138	6,212
Pre-modification Outstanding Recorded Investment	$ 387	$ 425
Post-modification Outstanding Recorded Investment	$ 306	$ 347
Prior-Year accruing
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	3,881	3,590
Pre-modification Outstanding Recorded Investment	$ 187	$ 211
Post-modification Outstanding Recorded Investment	$ 138	$ 164
Prior-Year accruing | Commercial Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	5	7
Pre-modification Outstanding Recorded Investment	$ 5	$ 30
Post-modification Outstanding Recorded Investment	$ 2	$ 25
Prior-Year accruing | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	3,876	3,583
Pre-modification Outstanding Recorded Investment	$ 182	$ 181
Post-modification Outstanding Recorded Investment	$ 136	$ 139
Nonperforming financial instruments
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	2,257	2,622
Pre-modification Outstanding Recorded Investment	$ 200	$ 214
Post-modification Outstanding Recorded Investment	$ 168	$ 183
Nonperforming financial instruments | Commercial Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	58	57
Pre-modification Outstanding Recorded Investment	$ 113	$ 136
Post-modification Outstanding Recorded Investment	$ 89	$ 111
Nonperforming financial instruments | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	2,199	2,565
Pre-modification Outstanding Recorded Investment	$ 87	$ 78
Post-modification Outstanding Recorded Investment	$ 79	$ 72
Commercial and industrial | Prior-Year accruing | Commercial Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	4	6
Pre-modification Outstanding Recorded Investment	$ 5	$ 30
Post-modification Outstanding Recorded Investment	$ 2	$ 25
Commercial and industrial | Nonperforming financial instruments | Commercial Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	51	51
Pre-modification Outstanding Recorded Investment	$ 51	$ 72
Post-modification Outstanding Recorded Investment	$ 39	$ 53
Commercial mortgage | Prior-Year accruing | Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	1	1
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	7	6
Pre-modification Outstanding Recorded Investment	$ 62	$ 64
Post-modification Outstanding Recorded Investment	$ 50	$ 58
Commercial real estate loans | Prior-Year accruing | Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	1	1
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	7	6
Pre-modification Outstanding Recorded Investment	$ 62	$ 64
Post-modification Outstanding Recorded Investment	$ 50	$ 58
Real estate — residential mortgage | Prior-Year accruing | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	472	493
Pre-modification Outstanding Recorded Investment	$ 35	$ 37
Post-modification Outstanding Recorded Investment	$ 29	$ 31
Real estate — residential mortgage | Nonperforming financial instruments | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	271	181
Pre-modification Outstanding Recorded Investment	$ 28	$ 13
Post-modification Outstanding Recorded Investment	$ 27	$ 11
Home equity loans | Prior-Year accruing | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	1,829	1,751
Pre-modification Outstanding Recorded Investment	$ 108	$ 104
Post-modification Outstanding Recorded Investment	$ 87	$ 84
Home equity loans | Nonperforming financial instruments | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	629	713
Pre-modification Outstanding Recorded Investment	$ 40	$ 42
Post-modification Outstanding Recorded Investment	$ 37	$ 41
Consumer direct loans | Prior-Year accruing | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	178	139
Pre-modification Outstanding Recorded Investment	$ 4	$ 4
Post-modification Outstanding Recorded Investment	$ 2	$ 3
Consumer direct loans | Nonperforming financial instruments | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	153	172
Pre-modification Outstanding Recorded Investment	$ 2	$ 2
Post-modification Outstanding Recorded Investment	$ 2	$ 2
Credit cards | Prior-Year accruing | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	584	486
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	279	368
Pre-modification Outstanding Recorded Investment	$ 2	$ 2
Post-modification Outstanding Recorded Investment	$ 1	$ 2
Consumer indirect loans | Prior-Year accruing | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	813	714
Pre-modification Outstanding Recorded Investment	$ 31	$ 33
Post-modification Outstanding Recorded Investment	$ 17	$ 20
Consumer indirect loans | Nonperforming financial instruments | Consumer Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | SecurityLoan	867	1,131
Pre-modification Outstanding Recorded Investment	$ 15	$ 19
Post-modification Outstanding Recorded Investment	$ 12	$ 16