Loans - Summary of Nonaccrual Loans and Accruing Receivables 90 Days or More Delinquent (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	$ 720	$ 602
Accruing Loans Contractually Past Due 90 Days or More	151	6
Nonaccrual Loans With No Allowance For Credit Losses	139	205
Residential mortgage loans carried at lower of cost or fair value of the collateral less cost to sell	106	108
Commercial loans
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	541	424
Accruing Loans Contractually Past Due 90 Days or More	147	1
Nonaccrual Loans With No Allowance For Credit Losses	54	119
Commercial loans | Real estate, including construction
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	261	229
Accruing Loans Contractually Past Due 90 Days or More	0	0
Nonaccrual Loans With No Allowance For Credit Losses	21	40
Commercial loans | Business and corporate banking
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	90	143
Accruing Loans Contractually Past Due 90 Days or More	61	1
Nonaccrual Loans With No Allowance For Credit Losses	17	29
Commercial loans | Global banking
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	190	49
Accruing Loans Contractually Past Due 90 Days or More	86	0
Nonaccrual Loans With No Allowance For Credit Losses	16	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	179	178
Accruing Loans Contractually Past Due 90 Days or More	4	5
Nonaccrual Loans With No Allowance For Credit Losses	85	86
Consumer loans | Residential mortgages
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	173	172
Accruing Loans Contractually Past Due 90 Days or More	0	0
Nonaccrual Loans With No Allowance For Credit Losses	80	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	3	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	1	2
Nonaccrual Loans With No Allowance For Credit Losses	$ 0	$ 0