Loans - Loans Classified as TDR Loans (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Financing Receivable, Modifications [Line Items]
Loans which were classified as TDR loans during the previous 12 months which became contractually delinquent	$ 14	$ 10	$ 36	$ 39
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Threshold period for troubled debt restructuring status	90 days	90 days	90 days	90 days
Loans which were classified as TDR loans during the previous 12 months which became contractually delinquent	$ 7	$ 0	$ 12	$ 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	0	0	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	$ 7	$ 0	$ 10	$ 0
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Threshold period for troubled debt restructuring status	60 days	60 days	60 days	60 days
Loans which were classified as TDR loans during the previous 12 months which became contractually delinquent	$ 7	$ 10	$ 24	$ 27
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	6	10	23	27
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Loans which were classified as TDR loans during the previous 12 months which became contractually delinquent	$ 1	$ 0	$ 1	$ 0