Loans - Summary of Nonaccrual Loans and Accruing Receivables 90 Days or More Delinquent (Detail) - USD ($) $ in Millions		Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		$ 1,643	$ 1,686
Accruing Loans Contractually Past Due 90 days or More		31	21
Nonaccrual Loans With No Allowance For Credit Losses		369	400
Residential mortgage loans carried at lower of cost or fair value of the collateral less cost to sell		371	375
Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		478	544
Accruing Loans Contractually Past Due 90 days or More		0	0
Nonaccrual Loans With No Allowance For Credit Losses		105	125
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		54	44
Accruing Loans Contractually Past Due 90 days or More		0	0
Nonaccrual Loans With No Allowance For Credit Losses		39	31
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		172	163
Accruing Loans Contractually Past Due 90 days or More		0	0
Nonaccrual Loans With No Allowance For Credit Losses		19	1
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		252	337
Accruing Loans Contractually Past Due 90 days or More		0	0
Nonaccrual Loans With No Allowance For Credit Losses		47	93
Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		1,165	1,142
Accruing Loans Contractually Past Due 90 days or More		31	21
Nonaccrual Loans With No Allowance For Credit Losses		264	275
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	[1],[2],[3]	1,101	1,079
Accruing Loans Contractually Past Due 90 days or More	[1],[2],[3]	0	0
Nonaccrual Loans With No Allowance For Credit Losses	[1],[2],[3]	231	241
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	[1],[3]	64	63
Accruing Loans Contractually Past Due 90 days or More	[1],[3]	0	0
Nonaccrual Loans With No Allowance For Credit Losses	[1],[3]	33	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		28	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		3	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		$ 535	$ 590

[1] At March 31, 2021 and December 31, 2020, nonaccrual consumer mortgage loans include $371 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.

[2] 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.

[3] 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 March 31, 2021 and December 31, 2020, nonaccrual consumer mortgage loans also included $535 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.