Financial Instruments (Narrative) (Detail) gal in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) gal	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) gal	Sep. 30, 2016 USD ($)
Schedule of Cost-method Investments [Line Items]
Document Period End Date			Sep. 30, 2017
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	$ 2		$ 2
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(23)	$ 6	(90)	$ 1
Interest Rate Caps [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	11,035		11,035
Interest Rate Caps [Member] | Subsidiaries [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	5,000		5,000
Interest Rate Swaps [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	1,100		1,100
Currency Swap [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	$ 887		$ 887
Commodity Contracts [Member]
Schedule of Cost-method Investments [Line Items]
Units Of Gasoline | gal	4		4
Purchase [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	$ 3,000		$ 3,000
Sold [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	8,000		8,000
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	0	(1)	0
Interest Expense [Member]
Schedule of Cost-method Investments [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	7	8	25	43
Operating Expense [Member]
Schedule of Cost-method Investments [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	$ 4	$ 3	$ 19	$ 26