Loans and Allowance for Loan Losses - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans 90 days or more past due and still accruing	$ 7	$ 1
Total loans held for investment	65,843	60,034
Commitment to lend additional funds to borrowers with loan modifications classified as TDRs	42
TDR modifications charge-offs	0
Minimum defaulting period	60
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Transfer of loans from held for investment to held for sale	215
Loans receivable sold during the period	199
FDIC Covered Loan [Member] | Commercial portfolio [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	302	365
FDIC Covered Loan [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	46	57
Purchased Credit Impaired [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans 90 days or more past due and still accruing	$ 210	$ 124