Allowance for Credit Losses - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Minimum delinquency period for loan level collectability analysis consumer mortgage	150 days
Residential Mortgage Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value of loans for which partial charge-off has been recognized	$ 34	$ 44
Mortgage loans on real real estate not requiring charge off due to collateral carrying amount	20	16
Home Equity Loans and Lines of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value of loans for which partial charge-off has been recognized	25	32
Mortgage loans on real real estate not requiring charge off due to collateral carrying amount	$ 32	$ 39