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Summary of Option Activity Under Company's Equity Incentive Plans (Detail) - $ / shares
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Number of Shares Underlying Outstanding Options    
Number of Shares Underlying Outstanding Options, Beginning Balance 821,741  
Number of Shares Underlying Outstanding Options, Options granted 227,900  
Number of Shares Underlying Outstanding Options, Options exercised (59)  
Number of Shares Underlying Outstanding Options, Options canceled/forfeited (23,189)  
Number of Shares Underlying Outstanding Options, Ending Balance 1,026,393 821,741
Number of Shares Underlying Outstanding Options, Exercisable - March 31, 2018 228,386  
Number of Shares Underlying Outstanding Options, Vested and expected to vest - March 31, 2018 1,026,393  
Weighted- Average Exercise Price    
Weighted- Average Exercise Price, Beginning Balance $ 13.69  
Weighted- Average Exercise Price, Options granted 14.83  
Weighted- Average Exercise Price, Options exercised 4.63  
Weighted- Average Exercise Price, Options Canceled/forfeited 4.46  
Weighted- Average Exercise Price, Ending Balance 14.15 $ 13.69
Weighted- Average Exercise Price, Exercisable - March 31, 2018 34.41  
Weighted- Average Exercise Price, Vested and expected to vest - March 31, 2018 $ 14.15  
Weighted-Average Remaining Contractual Term (Years)    
Weighted-Average Remaining Contractual Term (Years), Outstanding Balance 9 years 10 days 9 years 2 months 1 day
Weighted-Average Remaining Contractual Term (Years), Exercisable - March 31, 2018 7 years 8 months 19 days  
Weighted-Average Remaining Contractual Term (Years), Vested and expected to vest - March 31, 2018 9 years 10 days