Loans and Allowance for Credit Losses - Carrying Amount of Covered Assets (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Covered Assets [Line Items]
Purchased Impaired Loans	$ 1,913	$ 2,012
Purchased Nonimpaired Loans	463	551
Other	467	579
Covered loans	2,823	3,121
Total Covered Assets	2,843	3,142
Residential Mortgages [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	1,913	2,012
Purchased Nonimpaired Loans	353	400
Covered loans	2,266	2,412
Other Retail [Member]
Covered Assets [Line Items]
Purchased Nonimpaired Loans	110	151
Covered loans	110	151
Losses Reimbursable by FDIC [Member]
Covered Assets [Line Items]
Other	331	320
Covered loans	331	320
Unamortized Changes in FDIC Asset [Member]
Covered Assets [Line Items]
Other	116	238
Covered loans	116	238
Covered Loans [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	1,913	2,012
Purchased Nonimpaired Loans	463	551
Other	447	558
Covered loans	2,823	3,121
Foreclosed Real Estate [Member]
Covered Assets [Line Items]
Other	20	21
Total Covered Assets	$ 20	$ 21