Loans and Allowance for Credit Losses - Carrying Amount of Covered Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Covered Assets [Line Items]
Purchased Impaired Loans	$ 2,052	$ 2,248
Purchased Nonimpaired Loans	595	784
Other	641	830
Covered loans	3,262	3,836
Total Covered Assets	3,288	3,862
Residential Mortgages [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	2,052	2,248
Purchased Nonimpaired Loans	422	506
Covered loans	2,474	2,754
Other Retail [Member]
Covered Assets [Line Items]
Purchased Nonimpaired Loans	173	278
Covered loans	173	278
Losses Reimbursable by FDIC [Member]
Covered Assets [Line Items]
Other	320	381
Covered loans	320	381
Unamortized Changes in FDIC Asset [Member]
Covered Assets [Line Items]
Other	295	423
Covered loans	295	423
Covered Loans [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	2,052	2,248
Purchased Nonimpaired Loans	595	784
Other	615	804
Covered loans	3,262	3,836
Foreclosed Real Estate [Member]
Covered Assets [Line Items]
Other	26	26
Total Covered Assets	$ 26	$ 26