Derivative Financial Instruments - Notional Values of Derivative Contracts (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Notional value of derivative contracts, Total	$ 22,321	$ 26,060
Debt option [Member]
Derivatives, Fair Value [Line Items]
Interest rate derivatives associated with debt carried at fair value	8,707	8,707
Interest rate swaps [Member] | Debt option [Member]
Derivatives, Fair Value [Line Items]
Interest rate derivatives associated with debt carried at fair value	5,573	5,573
Currency swaps [Member] | Debt option [Member]
Derivatives, Fair Value [Line Items]
Interest rate derivatives associated with debt carried at fair value	3,134	3,134
Derivatives designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Total derivatives designated as hedging instruments	9,708	11,012
Derivatives designated as hedging instruments [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Interest rate derivatives designated as hedging instruments	3,645	4,949
Derivatives designated as hedging instruments [Member] | Currency swaps [Member]
Derivatives, Fair Value [Line Items]
Currency derivatives designated as hedging instruments	6,063	6,063
Derivatives not designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Total Non-qualifying hedges	3,906	6,341
Derivatives not designated as hedging instruments [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Interest rate derivatives not designated as hedging instruments	3,784	6,219
Derivatives not designated as hedging instruments [Member] | Currency swaps [Member]
Derivatives, Fair Value [Line Items]
Currency derivatives not designated as hedging instruments	$ 122	$ 122