Nonperforming Assets (Additional Information) (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Other Real Estate	$ 59	$ 57
Accrual Loans [Member]
Mortgage Loans in Process of Foreclosure, Amount	106	101
Proceeds due from FHA or VA [Member]
Mortgage Loans in Process of Foreclosure, Amount	99	97
Other Real Estate	43	45
Nonaccrual loans [Member]
Mortgage Loans in Process of Foreclosure, Amount	81	73
Residential Portfolio Segment [Member]
Other Real Estate	54	51
Commercial Portfolio Segment [Member]
Other Real Estate	3	4
Land and Land Improvements [Member]
Other Real Estate	$ 2	$ 2