Key Components of Convertible Debt (Detail) (Convertible Senior Notes 2011 and 2013, USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013
Convertible Senior Notes 2011 and 2013
Debt Instrument [Line Items]
Contractual interest expense on the coupon	$ 7	$ 15
Amortization of the discount component recognized as interest expense	16	31
Total interest expense on the convertible debt	$ 23	$ 46