Carrying Amounts and Fair Values of the Company's Financial Instruments (Parenthetical) (Detail) (USD $)	Sep. 30, 2012	Dec. 31, 2011
Estimated Fair Value of Loan Portfolio's Net Carrying Value, Percentage	101.00%	100.00%
Unfunded loan commitments and letters of credit	$ 42,000,000,000
Allowance for unfunded loan commitments and letters of credit	$ 51,000,000