Loans - Loans Classified as TDR Loans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Loans which were classified as TDR loans during previous 12 months which became contractually delinquent	$ 0	$ 0	$ 0
Threshold period for troubled debt restructuring status	90 days	90 days	90 days
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Loans which were classified as TDR loans during previous 12 months which became contractually delinquent	$ 11	$ 24	$ 37
Threshold period for troubled debt restructuring status	60 days	60 days	60 days
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Loans which were classified as TDR loans during previous 12 months which became contractually delinquent	$ 9	$ 24	$ 36
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Loans which were classified as TDR loans during previous 12 months which became contractually delinquent	$ 2	$ 0	$ 1