Financial Instruments (Narrative) (Detail) gal in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) gal	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) gal	Jun. 30, 2016 USD ($)
Schedule of Cost-method Investments [Line Items]
Document Period End Date			Jun. 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	(50)	$ 25	(67)	$ (4)
Interest Rate Caps [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	10,855		10,855
Interest Rate Caps [Member] | Subsidiaries [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	5,100		5,100
Interest Rate Swaps [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	1,800		1,800
Currency Swap [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	$ 1,176		$ 1,176
Commodity Contracts [Member]
Schedule of Cost-method Investments [Line Items]
Units Of Gasoline | gal	6		6
Purchase [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	$ 2,900		$ 2,900
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)	2	(2)	0
Interest Expense [Member]
Schedule of Cost-method Investments [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	11	26	18	35
Operating Expense [Member]
Schedule of Cost-method Investments [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	$ 10	$ 4	$ 15	$ 23