Derivative Financial Instruments - Notional Values of Derivative Contracts (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Total	$ 13,980	$ 16,467
Debt option [Member]
Derivatives, Fair Value [Line Items]
Total	6,574	7,235
Interest rate swaps [Member] | Debt option [Member]
Derivatives, Fair Value [Line Items]
Total	3,682	4,343
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	5,533
Derivatives designated as hedging instruments [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Total	1,959	3,256
Derivatives designated as hedging instruments [Member] | Currency swaps [Member]
Derivatives, Fair Value [Line Items]
Total	2,248	2,277
Derivatives not designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Total	3,199	3,699
Derivatives not designated as hedging instruments [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Total	3,199	3,699
Derivatives not designated as hedging instruments [Member] | Currency swaps [Member]
Derivatives, Fair Value [Line Items]
Total	$ 0	$ 0