Loans - Summary of Receivables which were Modified and as Result Became Classified as TDR Loans (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	$ 123	$ 210	$ 132	$ 354
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	110	190	110	304
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	24	190	24	304
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	86	0	86	0
Consumer Loans [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	13	20	22	50
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Loans which were modified and as a result became classified as TDR Loans	10	16	16	43
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	2	3	4	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	$ 1	$ 1	$ 2	$ 2