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Summary of Option Activity Under Company's Equity Incentive Plans (Detail) - $ / shares
9 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Number of Shares Underlying Outstanding Options    
Number of Shares Underlying Outstanding Options, Beginning Balance 1,361,977  
Number of Shares Underlying Outstanding Options, Options granted 426,500  
Number of Shares Underlying Outstanding Options, Options exercised (41,219)  
Number of Shares Underlying Outstanding Options, Options forfeited (166,722)  
Number of Shares Underlying Outstanding Options, Options expired (10,698)  
Number of Shares Underlying Outstanding Options, Ending Balance 1,569,838 1,361,977
Number of Shares Underlying Outstanding Options, Exercisable - June 30, 2019 649,116  
Number of Shares Underlying Outstanding Options, Vested and expected to vest - June 30, 2019 1,569,838  
Weighted- Average Exercise Price    
Weighted- Average Exercise Price, Beginning Balance $ 12.04  
Weighted- Average Exercise Price, Options granted 8.00  
Weighted- Average Exercise Price, Options exercised 4.57  
Weighted- Average Exercise Price, Options forfeited 8.27  
Weighted- Average Exercise Price, Options expired 92.53  
Weighted- Average Exercise Price, Ending Balance 10.99 $ 12.04
Weighted- Average Exercise Price, Exercisable - June 30, 2019 14.14  
Weighted- Average Exercise Price, Vested and expected to vest - June 30, 2019 $ 10.99  
Weighted-Average Remaining Contractual Term (Years)    
Weighted-Average Remaining Contractual Term (Years), Outstanding Balance 8 years 4 months 17 days 8 years 8 months 15 days
Weighted-Average Remaining Contractual Term (Years), Exercisable - June 30, 2019 7 years 10 months 9 days  
Weighted-Average Remaining Contractual Term (Years), Vested and expected to vest - June 30, 2019 8 years 4 months 17 days