Financial Instruments (Narrative) (Detail) gal in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) gal	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) gal	Sep. 30, 2014 USD ($)
Schedule of Cost-method Investments [Line Items]
Net unrealized gain (loss) on cash flow hedges	$ 1	$ 29	$ 25	$ 31
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	7		7
Derivative Instruments, Gain (Loss) Recognized in Income, Net	15	23	32	(7)
Interest Rate Caps [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	8,428		8,428
Interest Rate Caps [Member] | Interest Rate Caps Purchased [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	2,100		2,100
Interest Rate Caps [Member] | Interest Rate Caps Purchased [Member] | Subsidiary Issuers [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	4,200		4,200
Interest Rate Caps [Member] | Interest Rate Caps Sold [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	6,300		6,300
Interest Rate Swaps [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	$ 1,669		$ 1,669
Commodity Contracts [Member]
Schedule of Cost-method Investments [Line Items]
Units Of Gasoline | gal	6		6
Currency Swap [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	$ 724		$ 724
Not Designated As Hedging Instruments [Member] | Commodity Contracts [Member]
Schedule of Cost-method Investments [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(4)	(2)	0	(1)
Interest Expense [Member]
Schedule of Cost-method Investments [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	19	19	21	$ 7
Operating Expense [Member]
Schedule of Cost-method Investments [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	$ 2	$ 3	$ 16