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Available-for-Sale Investments - Summary of Available-for-Sale Securities Held (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 111,090 $ 111,542
Gross unrealized gains 118 5
Gross unrealized losses (4) (70)
Fair Value 111,204 111,477
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 36,175 53,549
Gross unrealized gains 30  
Gross unrealized losses   (47)
Fair Value 36,205 53,502
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 73,909 54,022
Gross unrealized gains 86 4
Gross unrealized losses (4) (23)
Fair Value 73,991 54,003
U.S. government agency securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 1,006 3,971
Gross unrealized gains 2 1
Fair Value $ 1,008 $ 3,972