Loans - Loans Classified as TDR Loans (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2014
Financing Receivable, Modifications [Line Items]
Loans which were classified as TDR loans during the previous 12 months which became contractually delinquent	$ 13	$ 12
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Threshold period for troubled debt restructuring status	90 days		90 days
Loans which were classified as TDR loans during the previous 12 months which became contractually delinquent	$ 2	3
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	2
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	$ 2	1
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Threshold period for troubled debt restructuring status	60 days		60 days
Loans which were classified as TDR loans during the previous 12 months which became contractually delinquent	$ 11	9
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	$ 11	$ 9