Loans - Loans Classified as TDR Loans (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Financing Receivable, Modifications [Line Items]
Loans which were classified as TDR loans during the previous 12 months which became contractually delinquent	$ 12	$ 20
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Loans which were classified as TDR loans during the previous 12 months which became contractually delinquent	3	12
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans which were classified as TDR loans during the previous 12 months which became contractually delinquent	2	12
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Modifications [Line Items]
Loans which were classified as TDR loans during the previous 12 months which became contractually delinquent	1	0
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Loans which were classified as TDR loans during the previous 12 months which became contractually delinquent	9	8
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Loans which were classified as TDR loans during the previous 12 months which became contractually delinquent	$ 9	$ 8