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Convertible Senior Notes - Summary of Net Carrying Amount of Liability Component of Notes (Detail) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Debt Disclosure [Abstract]    
Principal $ 460,000  
Unamortized discount (76,927)  
Unamortized issuance cost (6,490)  
Net carrying amount $ 376,583 $ 366,552