Loans - Summary of Receivables which were Modified and as Result Became Classified as TDR Loans (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Commercial loans removed from TDR classification	$ 12,000,000	$ 0	$ 0
Loans which were modified and as a result became classified as TDR Loans	12,000,000	41,000,000	247,000,000
Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	0	6,000,000	200,000,000
Commercial Loan [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	0	6,000,000	40,000,000
Commercial Loan [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	0	0	160,000,000
Consumer Loans [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	$ 12,000,000	$ 35,000,000	$ 47,000,000
Weighted average rate of reduction	2.18%	1.93%	1.91%
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	$ 8,000,000	$ 26,000,000	$ 34,000,000
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	1,000,000	5,000,000	9,000,000
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	$ 3,000,000	$ 4,000,000	$ 4,000,000