Financial Instruments (Narrative) (Detail) gal in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) gal	Mar. 31, 2017 USD ($)
Schedule of Cost-method Investments [Line Items]
Document Period End Date	Mar. 31, 2018
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	$ 4
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(16)	$ (17)
Interest Rate Caps [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	8,964
Interest Rate Caps [Member] | Subsidiaries [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	3,000
Interest Rate Swaps [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	1,000
Currency Swap [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	$ 1,356
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	$ 3,000
Sold [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	6,000
Not Designated As Hedging Instruments [Member] | Commodity Contracts [Member]
Schedule of Cost-method Investments [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0	(1)
Interest Expense [Member]
Schedule of Cost-method Investments [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	13	7
Operating Expense [Member]
Schedule of Cost-method Investments [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	$ 4	$ 5