Loans and Allowance for Loan Losses - Recorded Investment Amounts of Troubled Debt Restructurings (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans at date of default	$ 53	[1]	$ 110	[1]
Commercial and industrial [Member]
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans at date of default	3		10
Commercial mortgage [Member]
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans at date of default	1		77
Construction [Member]
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans at date of default	15		17
Commercial portfolio [Member]
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans at date of default	19		104
Residential mortgage [Member]
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans at date of default	34		6
Consumer portfolio [Member]
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans at date of default	$ 34

[1] Amounts exclude $6 million and $11 million of TDRs covered by FDIC loss share agreements at December 31, 2012 and 2011, respectively.