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Stock-Based Compensation - Summary of Restricted Stock Unit Activity (Detail) - RSUs [Member] - $ / shares
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Number of Awards    
Number of Awards, Beginning balance (in shares) 169,266  
Granted (in shares) 186,089  
Vested (in shares) (66,138)  
Canceled (in shares) (15,867)  
Number of Awards, Ending balance (in shares) 273,350 169,266
Weighted Average Grant Date Fair Value    
Weighted Average Grant Date Fair Value, Beginning balance (in dollars per share) $ 22.27  
Granted (in dollars per share) 15.60  
Vested (in dollars per share) 22.67  
Canceled (in dollars per share) 17.10  
Weighted Average Grant Date Fair Value, Ending balance (in dollars per share) $ 17.93 $ 22.27
Weighted Average Remaining Amortization Period (in Years) 1 year 9 months 18 days 1 year 7 months 6 days