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Available-for-Sale Investments - Summary of Available-for-Sale Securities Held (Detail) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 94,052 $ 111,542
Gross unrealized gains 42 5
Gross unrealized losses (3) (70)
Fair Value 94,091 111,477
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 90,062 107,571
Gross unrealized gains 42 4
Gross unrealized losses (3) (70)
Fair Value 90,101 107,505
U.S. government agency and treasury securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 3,990  
Fair Value $ 3,990  
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