Loans - Summary of Nonaccrual Loans and Accruing Receivables 90 Days or More Delinquent (Detail) - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		$ 1,196	$ 664
Accruing Loans Contractually Past Due 90 days or More		22	30
Nonaccrual Loans With No Allowance For Credit Losses		365	428
Residential mortgage loans carried at lower of cost or fair value of the collateral less cost to sell		324	289
Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		601	237
Accruing Loans Contractually Past Due 90 days or More		1	1
Nonaccrual Loans With No Allowance For Credit Losses		127	139
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		13	6
Accruing Loans Contractually Past Due 90 days or More		0	0
Nonaccrual Loans With No Allowance For Credit Losses		2	3
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		225	82
Accruing Loans Contractually Past Due 90 days or More		1	1
Nonaccrual Loans With No Allowance For Credit Losses		10	19
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		363	149
Accruing Loans Contractually Past Due 90 days or More		0	0
Nonaccrual Loans With No Allowance For Credit Losses		115	117
Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		595	427
Accruing Loans Contractually Past Due 90 days or More		21	29
Nonaccrual Loans With No Allowance For Credit Losses		238	289
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	[1],[2],[3]	553	381
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]	207	257
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	[1],[3]	42	46
Accruing Loans Contractually Past Due 90 days or More	[1],[3]	0	0
Nonaccrual Loans With No Allowance For Credit Losses	[1],[3]	31	32
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		19	24
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		2	5
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		$ 24

[1] At September 30, 2020 and December 31, 2019, nonaccrual consumer mortgage loans include $324 million and $289 million, respectively, of loans that are carried at the lower of amortized cost or fair value of the collateral less cost to sell. The decrease in nonaccrual consumer mortgage loans with no allowance for credit losses at September 30, 2020 reflects the impact of adopting new accounting guidance which requires expected recoveries related to subsequent increases in the fair value of collateral for collateral-dependent loans to be recognized in the allowance for credit losses beginning January 1, 2020. See Note 21, "New Accounting Pronouncements," for additional discussion.

[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 September 30, 2020, nonaccrual consumer mortgage loans also include $24 million of loans under COVID-19 related payment deferral programs where the borrowers were provided with extended payment deferral relief of more than six months and, as a result, have been placed on nonaccrual status.