Loans - Loans Classified as TDR Loans (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Commercial Loans [Member]
Financing Receivable, Troubled Debt Restructuring [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, Troubled Debt Restructuring [Line Items]
Loans which were classified as TDR loans during previous 12 months which became contractually delinquent	$ 0	$ 4	$ 3	$ 8
Threshold period for troubled debt restructuring status	60 days	60 days	60 days	60 days
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans which were classified as TDR loans during previous 12 months which became contractually delinquent	$ 0	$ 4	$ 3	$ 6
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans which were classified as TDR loans during previous 12 months which became contractually delinquent	$ 0	$ 0	$ 0	$ 2