Financial Instruments (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012 gal
Schedule of Cost-method Investments [Line Items]
Losses deferred in accumulated other comprehensive income	3
Fair value of derivatives that requires cash posted as collateral	4
Interest Rate Caps [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	7,900
Interest Rate Caps [Member] | Interest Rate Caps Purchased [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	1,300
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	5,300
Interest Rate Caps [Member] | Interest Rate Caps Sold [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	6,600
Interest Rate Swaps [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	625
Foreign Exchange Contracts [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	194
Foreign Exchange Swaps [Member]
Schedule of Cost-method Investments [Line Items]
Absolute notional values of derivative instruments	832
Commodity Contracts [Member]
Schedule of Cost-method Investments [Line Items]
Units of gasoline	5,000,000