Loans and Related Allowance for Credit Losses (Financial Impact and TDRs by Concession Type) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($) loan	Dec. 31, 2018 USD ($) loan
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan	12,343	14,623	12,181
Pre-TDR Amortized Cost Basis	$ 691
Post-TDR Amortized Cost Basis	602
Pre-TDR Recorded Investment		$ 450	$ 435
Post-TDR Recorded Investment		413	398
Principal Forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	39
Post-TDR Recorded Investment			3
Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	144
Post-TDR Recorded Investment		108	153
Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	$ 419
Post-TDR Recorded Investment		$ 305	$ 242
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan	73	75	85
Pre-TDR Amortized Cost Basis	$ 513
Post-TDR Amortized Cost Basis	441
Pre-TDR Recorded Investment		$ 278	$ 272
Post-TDR Recorded Investment		252	248
Commercial | Principal Forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	39
Post-TDR Recorded Investment			2
Commercial | Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	56
Post-TDR Recorded Investment		11	67
Commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	$ 346
Post-TDR Recorded Investment		$ 241	$ 179
Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans | loan	12,270	14,548	12,096
Pre-TDR Amortized Cost Basis	$ 178
Post-TDR Amortized Cost Basis	161
Pre-TDR Recorded Investment		$ 172	$ 163
Post-TDR Recorded Investment		161	150
Consumer | Principal Forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Recorded Investment			1
Consumer | Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	88
Post-TDR Recorded Investment		97	86
Consumer | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-TDR Amortized Cost Basis	$ 73
Post-TDR Recorded Investment		$ 64	$ 63