Loans - Loans Classified as TDR Loans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014
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	$ 10	$ 11	$ 24
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	$ 8	$ 9	$ 24
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	$ 2	$ 0