Loans and Allowance for Credit Losses - Carrying Amount of Covered Assets (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Covered Assets [Line Items]
Purchased Impaired Loans	$ 2,453	$ 2,502
Purchased Nonimpaired Loans	999	1,062
Other	1,010	1,064
Covered loans	4,429	4,596
Total Covered Assets	4,462	4,628
Residential Mortgages [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	2,453	2,502
Purchased Nonimpaired Loans	583	615
Covered loans	3,036	3,117
Other Retail [Member]
Covered Assets [Line Items]
Purchased Nonimpaired Loans	416	447
Covered loans	416	447
Losses Reimbursable by FDIC [Member]
Covered Assets [Line Items]
Other	394	517
Covered loans	394	517
Unamortized Changes in FDIC Asset [Member]
Covered Assets [Line Items]
Other	583	515
Covered loans	583	515
Covered Loans [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	2,453	2,502
Purchased Nonimpaired Loans	999	1,062
Other	977	1,032
Covered loans	4,429	4,596
Foreclosed Real Estate [Member]
Covered Assets [Line Items]
Other	33	32
Total Covered Assets	$ 33	$ 32