Fair Value Measurement- Summary of Carrying Amount of Hedged Financial Instruments, Related Notional Amount of Hedge and Estimated Fair Value of Derivatives (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Interest-bearing deposits with banks
Derivative [Line Items]
Carrying amount	$ 6,932	$ 11,328
Notional amount of hedge	6,932	11,328
Unrealized Gain	88	38
Unrealized (Loss)	(27)	(224)
Available-for-sale
Derivative [Line Items]
Carrying amount	6,152	5,597
Notional amount of hedge	6,088	5,355
Unrealized Gain	101	12
Unrealized (Loss)	(246)	(339)
Deposits
Derivative [Line Items]
Carrying amount	10	10
Notional amount of hedge	10	10
Unrealized Gain	1	1
Unrealized (Loss)	0	0
Long-term Debt
Derivative [Line Items]
Carrying amount	15,088	15,100
Notional amount of hedge	14,414	14,314
Unrealized Gain	795	911
Unrealized (Loss)	$ (5)	$ (4)