Stockholders' Equity (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended			3 Months Ended	6 Months Ended		3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2012 Company's Vehicle-Backed Debt And Floating Rate Debt [Member]	Jun. 30, 2013 Company's Vehicle-Backed Debt And Floating Rate Debt [Member]	Jun. 30, 2012 Company's Vehicle-Backed Debt And Floating Rate Debt [Member]	Jun. 30, 2013 Floating rate vehicle-backed and corporate debt [Member]	Jun. 30, 2013 Floating rate vehicle-backed and corporate debt [Member]	Jun. 30, 2012 Floating rate vehicle-backed and corporate debt [Member]	Jun. 30, 2013 3 1/2% Convertible Notes [Member]	Jun. 30, 2012 3 1/2% Convertible Notes [Member]	Jun. 30, 2013 Convertible Notes [Member] 3 1/2% Convertible Notes [Member]
Stockholders Equity [Line Items]
Senior notes interest percentage										3.50%	3.50%	3.50%
Repurchased warrants for the purchase of the Company's common stock	$ 29	$ 26
Proceeds from sale of convertible note hedge	40	38
Change in net purchase and issuance of shares										3
Net unrealized gain (losses) on cash flow hedges, before tax				4	1	11	7	1	19
Net unrealized gains (losses) on cash flow hedges	$ 1		$ 0