Debt - Key Components of Convertible Debt (Detail) (Convertible Senior Notes 2011 and 2013, USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013
Convertible Senior Notes 2011 and 2013
Debt Instrument [Line Items]
Contractual interest expense on the coupon	$ 7	$ 22
Amortization of the discount component recognized as interest expense	17	49
Total interest expense on the convertible debt	$ 24	$ 71