Loans - Summary of Nonaccrual Loans and Accruing Receivables 90 Days or More Delinquent (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	$ 761	$ 602
Accruing Loans Contractually Past Due 90 Days or More	4	6
Nonaccrual Loans With No Allowance For Credit Losses	136	205
Residential mortgage loans carried at lower of cost or fair value of the collateral less cost to sell	107	108
Commercial loans
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	593	424
Accruing Loans Contractually Past Due 90 Days or More	0	1
Nonaccrual Loans With No Allowance For Credit Losses	49	119
Commercial loans | Real estate, including construction
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	304	229
Accruing Loans Contractually Past Due 90 Days or More	0	0
Nonaccrual Loans With No Allowance For Credit Losses	0	40
Commercial loans | Business and corporate banking
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	59	143
Accruing Loans Contractually Past Due 90 Days or More	0	1
Nonaccrual Loans With No Allowance For Credit Losses	6	29
Commercial loans | Global banking
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	230	49
Accruing Loans Contractually Past Due 90 Days or More	0	0
Nonaccrual Loans With No Allowance For Credit Losses	43	48
Commercial loans | Other commercial
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	3
Accruing Loans Contractually Past Due 90 Days or More	0	0
Nonaccrual Loans With No Allowance For Credit Losses	0	2
Consumer loans
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	168	178
Accruing Loans Contractually Past Due 90 Days or More	4	5
Nonaccrual Loans With No Allowance For Credit Losses	87	86
Consumer loans | Residential mortgages
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	162	172
Accruing Loans Contractually Past Due 90 Days or More	0	0
Nonaccrual Loans With No Allowance For Credit Losses	82	81
Consumer loans | Home equity mortgages
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	6	6
Accruing Loans Contractually Past Due 90 Days or More	0	0
Nonaccrual Loans With No Allowance For Credit Losses	5	5
Consumer loans | Credit cards
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	0
Accruing Loans Contractually Past Due 90 Days or More	2	3
Nonaccrual Loans With No Allowance For Credit Losses	0	0
Consumer loans | Other consumer
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	0
Accruing Loans Contractually Past Due 90 Days or More	2	2
Nonaccrual Loans With No Allowance For Credit Losses	$ 0	$ 0