Nonperforming Assets (Additional Information) (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Other Real Estate	$ 53	$ 57
Accrual Loans [Member]
Mortgage Loans in Process of Foreclosure, Amount	107	101
Proceeds due from FHA or VA [Member]
Mortgage Loans in Process of Foreclosure, Amount	98	97
Other Real Estate	44	45
Nonaccrual loans [Member]
Mortgage Loans in Process of Foreclosure, Amount	77	73
Residential Portfolio Segment [Member]
Other Real Estate	49	51
Commercial Portfolio Segment [Member]
Other Real Estate	2	4
Land and Land Improvements [Member]
Other Real Estate	$ 2	$ 2