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Convertible Senior Notes - Summary of Net Carrying Amount of Liability Component of Notes (Detail) - USD ($)
Sep. 30, 2019
Dec. 31, 2018
Debt Disclosure [Abstract]    
Principal $ 460,000,000  
Unamortized discount (72,168,000)  
Unamortized issuance cost (6,131,000)  
Net carrying amount $ 381,701,000 $ 366,552,000