Loans and Allowance for Credit Losses - Carrying Amount of Covered Assets (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Covered Assets [Line Items]
Purchased Impaired Loans	$ 2,114	$ 2,248
Purchased Nonimpaired Loans	649	784
Other	699	830
Covered loans	3,437	3,836
Total Covered Assets	3,462	3,862
Residential Mortgages [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	2,114	2,248
Purchased Nonimpaired Loans	446	506
Covered loans	2,560	2,754
Other Retail [Member]
Covered Assets [Line Items]
Purchased Nonimpaired Loans	203	278
Covered loans	203	278
Losses Reimbursable by FDIC [Member]
Covered Assets [Line Items]
Other	326	381
Covered loans	326	381
Unamortized Changes in FDIC Asset [Member]
Covered Assets [Line Items]
Other	348	423
Covered loans	348	423
Covered Loans [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	2,114	2,248
Purchased Nonimpaired Loans	649	784
Other	674	804
Covered loans	3,437	3,836
Foreclosed Real Estate [Member]
Covered Assets [Line Items]
Other	25	26
Total Covered Assets	$ 25	$ 26