Derivative Financial Instruments - Notional Values of Derivative Contracts (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Total	$ 18,971	$ 26,060
Debt option [Member]
Derivatives, Fair Value [Line Items]
Total	7,235	8,707
Interest rate swaps [Member] | Debt option [Member]
Derivatives, Fair Value [Line Items]
Total	4,343	5,573
Currency swaps [Member] | Debt option [Member]
Derivatives, Fair Value [Line Items]
Total	2,892	3,134
Derivatives designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Total	7,830	11,012
Derivatives designated as hedging instruments [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Total	3,582	4,949
Derivatives designated as hedging instruments [Member] | Currency swaps [Member]
Derivatives, Fair Value [Line Items]
Total	4,248	6,063
Derivatives not designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Total	3,906	6,341
Derivatives not designated as hedging instruments [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Total	3,784	6,219
Derivatives not designated as hedging instruments [Member] | Currency swaps [Member]
Derivatives, Fair Value [Line Items]
Total	$ 122	$ 122