Loans - Summary of Receivables which were Modified and as Result Became Classified as TDR Loans (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Financing Receivable, Impaired [Line Items]
Total	$ 75	$ 46
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Total	39	10
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Total	26	10
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Total	13	0
Consumer Loans [Member]
Financing Receivable, Impaired [Line Items]
Total	36	36
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Total	35	33
Consumer Loans [Member] | Home Equity Line of Credit [Member]
Financing Receivable, Impaired [Line Items]
Total	0	1
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Impaired [Line Items]
Total	$ 1	$ 2