Loans and Allowance for Credit Losses - Carrying Amount of Covered Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Covered Assets [Line Items]
Purchased Impaired Loans	$ 2,580	$ 2,784
Purchased Nonimpaired Loans	1,130	1,322
Other Assets	1,112	1,212
Covered loans	4,791	5,281
Total covered assets	4,822	5,318
Residential Mortgages [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	2,580	2,784
Purchased Nonimpaired Loans	$ 644	$ 738
Other Assets
Covered loans	$ 3,224	$ 3,522
Other Retail [Member]
Covered Assets [Line Items]
Purchased Impaired Loans
Purchased Nonimpaired Loans	$ 486	$ 584
Other Assets
Covered loans	$ 486	$ 584
Losses Reimbursable by FDIC [Member]
Covered Assets [Line Items]
Purchased Impaired Loans
Purchased Nonimpaired Loans
Other Assets	$ 696	$ 717
Covered loans	$ 696	$ 717
Unamortized Changes in FDIC asset [Member]
Covered Assets [Line Items]
Purchased Impaired Loans
Purchased Nonimpaired Loans
Other Assets	$ 385	$ 458
Covered loans	385	458
Covered Loans [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	2,580	2,784
Purchased Nonimpaired Loans	1,130	1,322
Other Assets	1,081	1,175
Covered loans	$ 4,791	$ 5,281
Foreclosed Real Estate [Member]
Covered Assets [Line Items]
Purchased Impaired Loans
Purchased Nonimpaired Loans
Other Assets	$ 31	$ 37
Total covered assets	$ 31	$ 37