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Available-for-Sale Investments - Summary of Available-for-Sale Securities Held (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 111,542 $ 28,593
Gross unrealized gains 5 1
Gross unrealized losses (70) (9)
Fair Value 111,477 28,585
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 107,571 28,593
Gross unrealized gains 4 1
Gross unrealized losses (70) (9)
Fair Value 107,505 $ 28,585
U.S. government agency securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 3,971  
Gross unrealized gains 1  
Fair Value $ 3,972