Loans and Allowance for Loan Losses - Summary of Troubled Debt Restructurings (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans	$ 592	$ 473
Commercial and Industrial [Member]
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans	173	151
Commercial mortgage [Member]
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans	112	104
Construction [Member]
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans	39	66
Commercial portfolio [Member]
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans	324	321
Residential mortgage [Member]
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans	244	142
Home equity and other consumer loans [Member]
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans	18	2
Consumer portfolio [Member]
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans	262	144
FDIC covered Loans [Member]
Financing Receivable, Modifications [Line Items]
Total troubled debt restructured loans	$ 6	$ 8