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Stock-Based Compensation - Summary of Stock Option Activity (Detail) - 2006 and 2014 Stock Option Plans and Inducement Plan [Member] - Stock Options [Member] - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Share-Based Compensation    
Number of Shares Outstanding, beginning of the period 4,241,833  
Number of Shares, Granted 908,250  
Number of Shares, Forfeited (47,394)  
Number of Shares, Cancelled (10,219)  
Number of Shares Outstanding, end of the period 5,092,470 4,241,833
Number of Shares, Exercisable 2,672,591  
Number of Shares, Vested or expected to vest 4,520,320  
Weighted-Average Exercise Price Per Share Outstanding, beginning of the period $ 6.98  
Weighted-Average Exercise Price Per Share, Granted 3.42  
Weighted-Average Exercise Price Per Share, Forfeited 6.84  
Weighted-Average Exercise Price Per Share, Cancelled 9.02  
Weighted-Average Exercise Price Per Share Outstanding, end of the period 6.34 $ 6.98
Weighted-Average Exercise Price Per Share, Exercisable 7.67  
Weighted-Average Exercise Price Per Share, Vested or expected to vest $ 6.56  
Weighted-Average Remaining Contractual Term, Outstanding 7 years 3 months 14 days 7 years 7 days
Weighted-Average Remaining Contractual Term, Exercisable 5 years 6 months 29 days  
Weighted-Average Remaining Contractual Term, Vested or expected to vest 7 years 10 days  
Aggregate Intrinsic Value, Outstanding, beginning of the period $ 471  
Aggregate Intrinsic Value Outstanding, end of the period 260 $ 471
Aggregate Intrinsic Value, Exercisable 260  
Aggregate Intrinsic Value, Vested or expected to vest $ 260