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Investments in Marketable Securities - Summary of Investments Classified As Available-For-Sale Securities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 134,772 $ 11,606
Gross Unrealized Losses (116) (19)
Aggregate Estimated Fair Value 134,656 11,587
Commercial paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 3,458  
Aggregate Estimated Fair Value 3,458  
Certificates of deposit [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost   249
Aggregate Estimated Fair Value   249
Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 131,314 11,357
Gross Unrealized Losses (116) (19)
Aggregate Estimated Fair Value $ 131,198 $ 11,338