Summary of Carrying Amount of Hedged Financial Instruments, Related Notional Amount of the Hedge and Estimated Fair Value of Derivatives (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Interest-bearing Deposits
Derivative [Line Items]
Carrying amount	$ 9,211	$ 8,789
Notional amount of hedge	9,211	8,789
Unrealized Gain	42	441
Unrealized (Loss)		(17)
Available-for-sale
Derivative [Line Items]
Carrying amount	4,642	4,354
Notional amount of hedge	4,540	4,009
Unrealized Gain	84
Unrealized (Loss)	(160)	(289)
Deposits
Derivative [Line Items]
Carrying amount	10	10
Notional amount of hedge	10	10
Unrealized Gain	1	1
Long-term Debt
Derivative [Line Items]
Carrying amount	15,925	15,048
Notional amount of hedge	15,233	14,262
Unrealized Gain	901	964
Unrealized (Loss)	$ (18)	$ (9)