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	Jun. 30, 2012	Dec. 31, 2011
Interest-bearing Deposits
Derivative [Line Items]
Carrying amount	$ 7,500	$ 8,789
Notional amount of hedge	7,500	8,789
Unrealized Gain	173	441
Unrealized (Loss)		(17)
Available-for-sale
Derivative [Line Items]
Carrying amount	5,317	4,354
Notional amount of hedge	4,919	4,009
Unrealized (Loss)	(450)	(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	16,239	15,048
Notional amount of hedge	15,396	14,262
Unrealized Gain	988	964
Unrealized (Loss)	$ (8)	$ (9)