Loans and Related Allowance for Credit Losses (Financial Impact and TDRs by Concession Type) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) loan	Sep. 30, 2020 USD ($) loan	Sep. 30, 2021 USD ($) loan	Sep. 30, 2020 USD ($) loan
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan	1,353	2,785	4,865	9,983
Pre-TDR Amortized Cost Basis	$ 154	$ 141	$ 406	$ 443
Post-TDR Amortized Cost Basis	167	119	392	406
Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	21	36	49	77
Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	$ 146	83	343	290
Principal Forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis				$ 39
Total commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan	13	16	43	58
Pre-TDR Amortized Cost Basis	$ 123	$ 95	$ 320	$ 304
Post-TDR Amortized Cost Basis	139	79	315	280
Total commercial | Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis		10		10
Total commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	$ 139	$ 69	$ 315	231
Total commercial | Principal Forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis				$ 39
Total consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan	1,340	2,769	4,822	9,925
Pre-TDR Amortized Cost Basis	$ 31	$ 46	$ 86	$ 139
Post-TDR Amortized Cost Basis	28	40	77	126
Total consumer | Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	21	26	49	67
Total consumer | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	$ 7	$ 14	$ 28	$ 59