Derivative Financial Instruments - Notional Values of Derivative Contracts (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Total	$ 13,980	$ 13,980
Debt option [Member]
Derivatives, Fair Value [Line Items]
Total	6,574	6,574
Interest rate swaps [Member] | Debt option [Member]
Derivatives, Fair Value [Line Items]
Total	3,682	3,682
Currency swaps [Member] | Debt option [Member]
Derivatives, Fair Value [Line Items]
Total	2,892	2,892
Derivatives designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Total	4,207	4,207
Derivatives designated as hedging instruments [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Total	1,959	1,959
Derivatives designated as hedging instruments [Member] | Currency swaps [Member]
Derivatives, Fair Value [Line Items]
Total	2,248	2,248
Derivatives not designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Total	3,199	3,199
Derivatives not designated as hedging instruments [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Total	$ 3,199	$ 3,199