Loans - Loans Classified as TDR Loans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Commercial Loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Threshold period for troubled debt restructuring status	90 days	90 days	90 days
Loans which were classified as TDR loans during previous 12 months which became contractually delinquent	$ 0	$ 0	$ 0
Consumer Loans [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Threshold period for troubled debt restructuring status	60 days	60 days	60 days
Loans which were classified as TDR loans during previous 12 months which became contractually delinquent	$ 5	$ 3	$ 10
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	5	3	8
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	$ 2