Carrying Amounts and Fair Values of the Company's Financial Instruments (Parenthetical) (Detail) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value of Loan Portfolio's Net Carrying Value, Percentage	100.00%	101.00%
Unfunded loan commitments and letters of credit	$ 44,300,000,000	$ 42,700,000,000
Allowance for unfunded loan commitments and letters of credit	$ 56,000,000	$ 49,000,000