Debt - Balances of Liability and Equity Components (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Liability component—principal amount	$ 548,500	$ 448,500
Unamortized debt discount	(37,690)	(31,968)
Liability component—net carrying amount	510,810	416,532
Equity component	$ 62,504	$ 61,314