Loans - Summary of Past Due Status of Loans With Payment Modifications (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Financing Receivable, Past Due [Line Items]
Modified loans, past due	$ 567	$ 443
30 - 89 Days
Financing Receivable, Past Due [Line Items]
Modified loans, past due	2	1
90+ Days
Financing Receivable, Past Due [Line Items]
Modified loans, past due	13	29
Current
Financing Receivable, Past Due [Line Items]
Modified loans, past due	552	413
Commercial loans
Financing Receivable, Past Due [Line Items]
Modified loans, past due	528	438
Commercial loans | Real estate, including construction
Financing Receivable, Past Due [Line Items]
Modified loans, past due	318	232
Commercial loans | Business and corporate banking
Financing Receivable, Past Due [Line Items]
Modified loans, past due	210	206
Commercial loans | 30 - 89 Days
Financing Receivable, Past Due [Line Items]
Modified loans, past due	0	0
Commercial loans | 30 - 89 Days | Real estate, including construction
Financing Receivable, Past Due [Line Items]
Modified loans, past due	0	0
Commercial loans | 30 - 89 Days | Business and corporate banking
Financing Receivable, Past Due [Line Items]
Modified loans, past due	0	0
Commercial loans | 90+ Days
Financing Receivable, Past Due [Line Items]
Modified loans, past due	13	29
Commercial loans | 90+ Days | Real estate, including construction
Financing Receivable, Past Due [Line Items]
Modified loans, past due	0	0
Commercial loans | 90+ Days | Business and corporate banking
Financing Receivable, Past Due [Line Items]
Modified loans, past due	13	29
Commercial loans | Current
Financing Receivable, Past Due [Line Items]
Modified loans, past due	515	409
Commercial loans | Current | Real estate, including construction
Financing Receivable, Past Due [Line Items]
Modified loans, past due	318	232
Commercial loans | Current | Business and corporate banking
Financing Receivable, Past Due [Line Items]
Modified loans, past due	197	177
Consumer loans
Financing Receivable, Past Due [Line Items]
Modified loans, past due	39	5
Consumer loans | Residential mortgages
Financing Receivable, Past Due [Line Items]
Modified loans, past due	39	5
Consumer loans | 30 - 89 Days
Financing Receivable, Past Due [Line Items]
Modified loans, past due	2	1
Consumer loans | 30 - 89 Days | Residential mortgages
Financing Receivable, Past Due [Line Items]
Modified loans, past due	2	1
Consumer loans | 90+ Days
Financing Receivable, Past Due [Line Items]
Modified loans, past due	0	0
Consumer loans | 90+ Days | Residential mortgages
Financing Receivable, Past Due [Line Items]
Modified loans, past due	0	0
Consumer loans | Current
Financing Receivable, Past Due [Line Items]
Modified loans, past due	37	4
Consumer loans | Current | Residential mortgages
Financing Receivable, Past Due [Line Items]
Modified loans, past due	$ 37	$ 4