Derivative Financial Instruments - Notional Values of Derivative Contracts (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Total	$ 1,765	$ 8,933
Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Total	0	1,859
Currency swaps [Member]
Derivatives, Fair Value [Line Items]
Total	1,562	1,562
Debt Option [Member]
Derivatives, Fair Value [Line Items]
Total	1,562	3,421
Derivatives designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Total	203	2,888
Derivatives designated as hedging instruments [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Total	0	1,300
Derivatives designated as hedging instruments [Member] | Currency swaps [Member]
Derivatives, Fair Value [Line Items]
Total	203	1,588
Derivatives not designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Total	0	2,624
Derivatives not designated as hedging instruments [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Total	$ 0	$ 2,624