Debt - Balances of Liability and Equity Components (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Debt Disclosure [Abstract]
Liability component—principal amount	$ 448,500	$ 448,500
Unamortized debt discount	(44,255)	(51,202)
Liability component—net carrying amount	404,245	397,298
Equity component	$ 57,428	$ 55,236