Loans - Loans Classified as TDR Loans (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
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	$ 0
Threshold period for troubled debt restructuring status	90 days	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	$ 4	$ 3	$ 8	$ 8
Threshold period for troubled debt restructuring status	60 days	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	$ 4	$ 2	$ 6	$ 7
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	$ 0	$ 1	$ 2	$ 1