Commitments and Contingent Liabilities- Summary of Off-Balance Sheet Credit Risks, Net of Participations (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Lending commitments
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet credit risks participations	$ 612	$ 350
Standby letters of credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Off-balance sheet credit risks participations	$ 1,000	$ 1,000