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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