Loans - Summary of Nonaccrual Loans and Accruing Receivables 90 Days or More Delinquent (Detail) - USD ($) $ in Millions		Sep. 30, 2021		Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		$ 1,231			$ 1,686
Accruing Loans Contractually Past Due 90 days or More		1			21
Nonaccrual Loans With No Allowance For Credit Losses		438			400
Residential mortgage loans carried at lower of cost or fair value of the collateral less cost to sell		323			375
Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		399			544
Accruing Loans Contractually Past Due 90 days or More		1			0
Nonaccrual Loans With No Allowance For Credit Losses		198			125
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		128			44
Accruing Loans Contractually Past Due 90 days or More		0			0
Nonaccrual Loans With No Allowance For Credit Losses		35			31
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		139			163
Accruing Loans Contractually Past Due 90 days or More		1			0
Nonaccrual Loans With No Allowance For Credit Losses		73			1
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		132			337
Accruing Loans Contractually Past Due 90 days or More		0			0
Nonaccrual Loans With No Allowance For Credit Losses		90			93
Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		832	[1]		1,142
Accruing Loans Contractually Past Due 90 days or More		0	[1]		21
Nonaccrual Loans With No Allowance For Credit Losses		240	[1]		275
Consumer Loans [Member] | Loans Held for Sale [Member]
Financing Receivable, Past Due [Line Items]
Accruing Loans Contractually Past Due 90 days or More				$ 21
Consumer Loans [Member] | Loans Held for Sale [Member] | Exit of Mass Market Retail Banking Business and Remaining Retail Credit Card Portfolio [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans				$ 206
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	[2],[3],[4]	785			1,079
Accruing Loans Contractually Past Due 90 days or More	[2],[3],[4]	0			0
Nonaccrual Loans With No Allowance For Credit Losses	[2],[3],[4]	214			241
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	[2],[4]	47			63
Accruing Loans Contractually Past Due 90 days or More	[2],[4]	0			0
Nonaccrual Loans With No Allowance For Credit Losses	[2],[4]	26			34
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		0			0
Accruing Loans Contractually Past Due 90 days or More		0			19
Nonaccrual Loans With No Allowance For Credit Losses		0			0
Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		0			0
Accruing Loans Contractually Past Due 90 days or More		0			2
Nonaccrual Loans With No Allowance For Credit Losses		0			0
Consumer Loans [Member] | Mortgages [Member] | Cares Act Election [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		$ 367			$ 590

[1] The decrease in nonperforming loans at September 30, 2021 includes the impact of our decision to exit our mass market retail banking business as well as our remaining retail credit card portfolio which resulted in the transfer of certain loans to held for sale during the second quarter of 2021, including $206 million of nonperforming consumer loans, of which $21 million were accruing loans contractually 90 days or more past due.

[2] At September 30, 2021 and December 31, 2020, nonaccrual consumer mortgage loans include $323 million and $375 million, respectively, of loans that are carried at the lower of amortized cost or fair value of the collateral less cost to sell.

[3] Nonaccrual consumer mortgage loans for all periods does not include guaranteed loans purchased from the Government National Mortgage Association. Repayment of these loans is predominantly insured by the Federal Housing Administration and as such, these loans have different risk characteristics from the rest of our consumer loan portfolio.

[4] Nonaccrual consumer mortgage loans include all loans which are 90 or more days contractually delinquent as well as loans discharged under Chapter 7 bankruptcy and not re-affirmed and second lien loans where the first lien loan that we own or service is 90 or more days contractually delinquent. At September 30, 2021 and December 31, 2020, nonaccrual consumer mortgage loans also included $367 million and $590 million, respectively, of loans under COVID-19 related payment deferral programs where the borrowers utilized a payment deferral of more than six months and, as a result, have been placed on nonaccrual status.