Derivative Financial Instruments - Notional Values of Derivative Contracts (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Notional value of derivative contracts, Total	$ 26,060	$ 40,854
Debt option [Member]
Derivatives, Fair Value [Line Items]
Notional amount of derivatives associated with debt carried at fair value	8,707	13,076
Interest rate swaps [Member] | Debt option [Member]
Derivatives, Fair Value [Line Items]
Notional amount of derivatives associated with debt carried at fair value	5,573	9,700
Currency swaps [Member] | Debt option [Member]
Derivatives, Fair Value [Line Items]
Notional amount of derivatives associated with debt carried at fair value	3,134	3,376
Derivatives designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Notional Amount Of Derivative Designated As Hedge Instrument	11,012	16,335
Derivatives designated as hedging instruments [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Interest rate derivatives designated as hedging instruments	4,949	8,140
Derivatives designated as hedging instruments [Member] | Currency swaps [Member]
Derivatives, Fair Value [Line Items]
Currency derivatives designated as hedging instruments	6,063	8,195
Derivatives not designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Notional Amount Of Derivative Not Designated As Hedge Instrument	6,341	11,443
Derivatives not designated as hedging instruments [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Notional amount of interest rate derivatives not designated as hedging instruments for non-qualifying hedges	6,219	10,523
Derivatives not designated as hedging instruments [Member] | Currency swaps [Member]
Derivatives, Fair Value [Line Items]
Notional amount of foreign currency derivatives not designated as hedging instruments for non-qualifying economic hedges	122	715
Derivatives not designated as hedging instruments [Member] | Foreign exchange forward [Member]
Derivatives, Fair Value [Line Items]
Notional amount of foreign currency derivatives not designated as hedging instruments for non-qualifying economic hedges	$ 0	$ 205