Loans and Allowance for Credit Losses - Carrying Amount of Covered Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Covered Assets [Line Items]
Purchased Impaired Loans	$ 669	$ 2,012
Purchased Nonimpaired Loans	344	551
Other	1,702	579
Covered loans	1,400	3,121
Total Covered Assets	2,715	3,142
Residential Mortgages [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	669	2,012
Purchased Nonimpaired Loans	245	400
Covered loans	914	2,412
Other Retail [Member]
Covered Assets [Line Items]
Purchased Nonimpaired Loans	99	151
Covered loans	99	151
Losses Reimbursable by FDIC [Member]
Covered Assets [Line Items]
Other	329	320
Covered loans	329	320
Unamortized Changes in FDIC Asset [Member]
Covered Assets [Line Items]
Other	58	238
Covered loans	58	238
Covered Loans [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	669	2,012
Purchased Nonimpaired Loans	344	551
Other	387	558
Covered loans	1,400	3,121
Foreclosed Real Estate [Member]
Covered Assets [Line Items]
Other	19	21
Total Covered Assets	19	$ 21
Covered Loans Held For Sale [Member]
Covered Assets [Line Items]
Other	1,296
Covered loans	$ 1,296