Loans and Allowance for Credit Losses - Carrying Amount of Covered Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Covered Assets [Line Items]
Purchased Impaired Loans	$ 2,322	$ 2,502
Purchased Nonimpaired Loans	853	1,062
Other	874	1,064
Covered loans	4,021	4,596
Total Covered Assets	4,049	4,628
Residential Mortgages [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	2,322	2,502
Purchased Nonimpaired Loans	534	615
Covered loans	2,856	3,117
Other Retail [Member]
Covered Assets [Line Items]
Purchased Nonimpaired Loans	319	447
Covered loans	319	447
Losses Reimbursable by FDIC [Member]
Covered Assets [Line Items]
Other	375	517
Covered loans	375	517
Unamortized Changes in FDIC Asset [Member]
Covered Assets [Line Items]
Other	471	515
Covered loans	471	515
Covered Loans [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	2,322	2,502
Purchased Nonimpaired Loans	853	1,062
Other	846	1,032
Covered loans	4,021	4,596
Foreclosed Real Estate [Member]
Covered Assets [Line Items]
Other	28	32
Total Covered Assets	$ 28	$ 32