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Stock-Based Awards - Summary of Stock Option Activity (Detail) - USD ($)
$ / shares in Units, $ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]    
Shares issuable under options, beginning balance 5,024,371  
Shares issuable under options, granted 2,276,197  
Shares issuable under options, exercised (10,667)  
Shares issuable under options, forfeited (2,171,621)  
Shares issuable under options, ending balance 5,118,280 5,024,371
Shares issuable under options, options exercisable 2,701,812  
Weighted-average exercise price, beginning balance $ 8.85  
Weighted-average exercise price, granted 4.00  
Weighted-average exercise price, exercised 0.75  
Weighted-average exercise price, forfeited 7.60  
Weighted-average exercise price, ending balance 7.24 $ 8.85
Weighted-average exercise price, options exercisable $ 9.53  
Average remaining contractual life 6 years 4 months 24 days 7 years 7 months 6 days
Average remaining contractual life, options exercisable 4 years 2 months 12 days  
Aggregate intrinsic value   $ 3,143