Loans - Summary of Receivables which were Modified and as Result Became Classified as TDR Loans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	$ 224	$ 12	$ 41
Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	139	0	6
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	95	0	6
Commercial Loan [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	44	0	0
Consumer Loans [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	$ 85	$ 12	$ 35
Weighted average rate of reduction	1.75%	2.18%	1.93%
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	$ 78	$ 8	$ 26
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	3	1	5
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	$ 4	$ 3	$ 4