Loans and Allowance for Loan Losses - Summary of Loans (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred fees and costs of loans	$ 54	$ 5
FDIC Loss Share Agreements [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reimbursement of purchased credit impaired loan	348	421
FDIC Covered Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired not accounted as deteriorated credit quality	$ 21	$ 24