Loans and Allowance for Credit Losses - Carrying Amount of Covered Assets (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Covered Assets [Line Items]
Purchased Impaired Loans	$ 1,966	$ 2,012
Purchased Nonimpaired Loans	511	551
Other	522	579
Covered loans	2,979	3,121
Total Covered Assets	2,999	3,142
Residential Mortgages [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	1,966	2,012
Purchased Nonimpaired Loans	374	400
Covered loans	2,340	2,412
Other Retail [Member]
Covered Assets [Line Items]
Purchased Nonimpaired Loans	137	151
Covered loans	137	151
Losses Reimbursable by FDIC [Member]
Covered Assets [Line Items]
Other	327	320
Covered loans	327	320
Unamortized Changes in FDIC Asset [Member]
Covered Assets [Line Items]
Other	175	238
Covered loans	175	238
Covered Loans [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	1,966	2,012
Purchased Nonimpaired Loans	511	551
Other	502	558
Covered loans	2,979	3,121
Foreclosed Real Estate [Member]
Covered Assets [Line Items]
Other	20	21
Total Covered Assets	$ 20	$ 21