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Stock Plans - Summary of RSU Activity (Detail) - Restricted Stock Units [Member] - Amended and Restated 2014 Stock Option and Incentive Plan [Member] - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of Shares, Outstanding, Beginning Balance 117,500  
Number of Shares, Granted 232,750 117,500
Number of Shares, Vested and settled (56,406)  
Number of Shares, Cancelled (22,125)  
Number of Shares, Outstanding, Ending Balance 271,719 117,500
Weighted Average Grant Date Fair Value Per Share, Beginning Balance $ 26.00  
Weighted Average Grant Date Fair Value Per Share, Granted 6.40 $ 26.00
Weighted Average Grant Date Fair Value Per Share, Vested and settled 6.76  
Weighted Average Grant Date Fair Value Per Share, Cancelled 6.80  
Weighted Average Grant Date Fair Value Per Share, Ending Balance $ 4.20 $ 26.00