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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, 2012
Sep. 30, 2013
Sep. 30, 2012
Convertible Senior Notes 2011 and 2013
       
Debt Instrument [Line Items]        
Contractual interest expense on the coupon $ 7 $ 8 $ 22 $ 22
Amortization of the discount component recognized as interest expense 17 15 49 45
Total interest expense on the convertible debt $ 24 $ 23 $ 71 $ 67