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, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 304,307	$ 293,596
Accruing 30-89 Days Past Due	1,036	1,209
Accruing 90 Days or More Past Due	461	605
Nonperforming	1,181	692
Total loans	306,985	296,102
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	109,994	103,273
Accruing 30-89 Days Past Due	243	307
Accruing 90 Days or More Past Due	68	79
Nonperforming	459	204
Total loans	110,764	103,863
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	39,957	39,627
Accruing 30-89 Days Past Due	92	34
Accruing 90 Days or More Past Due	1	3
Nonperforming	330	82
Total loans	40,380	39,746
Residential Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	76,197	70,071
Accruing 30-89 Days Past Due	238	154
Accruing 90 Days or More Past Due	114	120
Nonperforming	240	241
Total loans	76,789	70,586
Other Retail [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	56,666	56,463
Accruing 30-89 Days Past Due	257	393
Accruing 90 Days or More Past Due	79	97
Nonperforming	152	165
Total loans	57,154	57,118
Credit Card [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	21,493	24,162
Accruing 30-89 Days Past Due	206	321
Accruing 90 Days or More Past Due	199	306
Total loans	$ 21,898	$ 24,789