Loans and Allowance for Credit Losses - Carrying Amount of Covered Assets (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Covered Assets [Line Items]
Purchased Impaired Loans	$ 2,728	$ 2,784
Purchased Nonimpaired Loans	1,267	1,322
Other Assets	1,160	1,212
Covered loans	5,118	5,281
Total covered assets	5,155	5,318
Residential Mortgages [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	2,728	2,784
Purchased Nonimpaired Loans	707	738
Other Assets
Covered loans	3,435	3,522
Other Retail [Member]
Covered Assets [Line Items]
Purchased Impaired Loans
Purchased Nonimpaired Loans	560	584
Other Assets
Covered loans	560	584
Losses Reimbursable by FDIC [Member]
Covered Assets [Line Items]
Purchased Impaired Loans
Purchased Nonimpaired Loans
Other Assets	703	717
Covered loans	703	717
Unamortized Changes in FDIC asset [Member]
Covered Assets [Line Items]
Purchased Impaired Loans
Purchased Nonimpaired Loans
Other Assets	420	458
Covered loans	420	458
Covered Loans [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	2,728	2,784
Purchased Nonimpaired Loans	1,267	1,322
Other Assets	1,123	1,175
Covered loans	5,118	5,281
Foreclosed Real Estate [Member]
Covered Assets [Line Items]
Purchased Impaired Loans
Purchased Nonimpaired Loans
Other Assets	37	37
Total covered assets	$ 37	$ 37