Loans and Allowance for Credit Losses - Carrying Amount of Covered Assets (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Covered Assets [Line Items]
Purchased Impaired Loans	$ 2,661	$ 2,784
Purchased Nonimpaired Loans	1,212	1,322
Other Assets	1,146	1,212
Covered loans	4,984	5,281
Total covered assets	5,019	5,318
Residential Mortgages [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	2,661	2,784
Purchased Nonimpaired Loans	$ 677	738
Other Assets
Covered loans	$ 3,338	3,522
Other Retail [Member]
Covered Assets [Line Items]
Purchased Impaired Loans
Purchased Nonimpaired Loans	$ 535	584
Other Assets
Covered loans	$ 535	584
Losses Reimbursable by FDIC [Member]
Covered Assets [Line Items]
Purchased Impaired Loans
Purchased Nonimpaired Loans
Other Assets	$ 705	717
Covered loans	$ 705	717
Unamortized Changes in FDIC asset [Member]
Covered Assets [Line Items]
Purchased Impaired Loans
Purchased Nonimpaired Loans
Other Assets	$ 406	458
Covered loans	406	458
Covered Loans [Member]
Covered Assets [Line Items]
Purchased Impaired Loans	2,661	2,784
Purchased Nonimpaired Loans	1,212	1,322
Other Assets	1,111	1,175
Covered loans	$ 4,984	5,281
Foreclosed Real Estate [Member]
Covered Assets [Line Items]
Purchased Impaired Loans
Purchased Nonimpaired Loans
Other Assets	$ 35	37
Total covered assets	$ 35	$ 37