Loans and Allowance for Credit Losses - Summary of Loans by Portfolio Class, Including Delinquency Status of those that Continue to Accrue Interest and are Nonperforming (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	$ 297,608	$ 297,707
Current [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	295,535	294,716
Accruing 30-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	782	1,290
Accruing 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	385	477
Nonperforming [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	906	1,224
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	101,013	102,871
Commercial [Member] | Current [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	100,595	102,127
Commercial [Member] | Accruing 30-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	163	314
Commercial [Member] | Accruing 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	39	55
Commercial [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	216	375
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	38,808	39,311
Commercial Real Estate [Member] | Current [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	38,459	38,676
Commercial Real Estate [Member] | Accruing 30-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	32	183
Commercial Real Estate [Member] | Accruing 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	21	2
Commercial Real Estate [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	296	450
Residential Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	74,954	76,155
Residential Mortgages [Member] | Current [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	74,450	75,529
Residential Mortgages [Member] | Accruing 30-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	153	244
Residential Mortgages [Member] | Accruing 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	114	137
Residential Mortgages [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	237	245
Other Retail [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	60,696	57,024
Other Retail [Member] | Current [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	60,224	56,466
Other Retail [Member] | Accruing 30-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	251	318
Other Retail [Member] | Accruing 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	64	86
Other Retail [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	157	154
Credit Card [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	22,137	22,346
Credit Card [Member] | Current [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	21,807	21,918
Credit Card [Member] | Accruing 30-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	183	231
Credit Card [Member] | Accruing 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	$ 147	$ 197