Financial Instruments (Narrative) (Detail) gal in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($) gal	Mar. 31, 2016 USD ($)
Schedule of Cost-method Investments [Line Items]
Document Period End Date	Mar. 31, 2017
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	$ 3
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(17)	$ (29)
Interest Rate Caps [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	9,836
Interest Rate Caps [Member] | Subsidiary Issuers [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	5,000
Interest Rate Swaps [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	2,050
Currency Swap [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	$ 1,133
Commodity Contracts [Member]
Schedule of Cost-method Investments [Line Items]
Units Of Gasoline | gal	8
Purchase [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	$ 2,400
Sold [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	7,400
Not Designated As Hedging Instruments [Member] | Commodity Contracts [Member]
Schedule of Cost-method Investments [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(1)	(2)
Interest Expense [Member]
Schedule of Cost-method Investments [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	7	9
Operating Expense [Member]
Schedule of Cost-method Investments [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	$ 5	$ 19