Summary of Off-Balance Sheet Credit Transactions, Net of Participations (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Lending commitments
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet credit risks participations	$ 304	$ 326
Standby letters of credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet credit risks participations	$ 1,100	$ 1,200