Loans and Related Allowance for Credit Losses (Financial Impact and TDRs by Concession Type) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) loan	Mar. 31, 2020 USD ($) loan
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan	2,115	3,580
Pre-TDR Amortized Cost Basis	$ 125	$ 98
Post-TDR Amortized Cost Basis	122	75
Principal Forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis		6
Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	16	22
Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	$ 106	$ 47
Total commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan	19	13
Pre-TDR Amortized Cost Basis	$ 93	$ 62
Post-TDR Amortized Cost Basis	94	43
Total commercial | Principal Forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis		6
Total commercial | Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis
Total commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	$ 94	$ 37
Total consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan	2,096	3,567
Pre-TDR Amortized Cost Basis	$ 32	$ 36
Post-TDR Amortized Cost Basis	28	32
Total consumer | Principal Forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis
Total consumer | Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	16	22
Total consumer | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	$ 12	$ 10