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Summary of Available-for-Sale Securities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 71,040 $ 95,166
Gross Unrealized Gain 139 2
Gross Unrealized Loss   (283)
Fair Value 71,179 94,885
Money market funds    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 10,493 6,167
Fair Value 10,493 6,167
United States government agency securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 14,511 15,971
Gross Unrealized Gain 8  
Gross Unrealized Loss   (23)
Fair Value 14,519 15,948
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 46,036 73,028
Gross Unrealized Gain 131 2
Gross Unrealized Loss   (260)
Fair Value $ 46,167 $ 72,770