Loans - Summary of Nonaccrual Loans and Accruing Receivables 90 Days or More Delinquent (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total nonaccruing loans	$ 664	$ 539
Total accruing loans contractually past due 90 days or more	30	21
Total nonperforming loans	694	560
Residential mortgage loans carried at lower of cost or fair value of the collateral less cost to sell	289	289
Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Total nonaccruing loans	237	143
Total accruing loans contractually past due 90 days or more	1	1
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Total nonaccruing loans	6	7
Total accruing loans contractually past due 90 days or more	0	0
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Total nonaccruing loans	82	70
Total accruing loans contractually past due 90 days or more	1	1
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Total nonaccruing loans	149	65
Total accruing loans contractually past due 90 days or more	0	0
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Past Due [Line Items]
Total nonaccruing loans	0	1
Total accruing loans contractually past due 90 days or more	0	0
Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Total nonaccruing loans	427	396
Total accruing loans contractually past due 90 days or more	29	20
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Total nonaccruing loans	381	341
Total accruing loans contractually past due 90 days or more	0	0
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Total nonaccruing loans	46	55
Total accruing loans contractually past due 90 days or more	0	0
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Past Due [Line Items]
Total nonaccruing loans	0	0
Total accruing loans contractually past due 90 days or more	24	14
Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Past Due [Line Items]
Total nonaccruing loans	0	0
Total accruing loans contractually past due 90 days or more	$ 5	$ 6