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Stock-Based Compensation - Summary of Stock-Based Compensation Activity (Detail) - 2008 Plan, 2015 Plan and 2018 Plan [Member] - USD ($)
$ / shares in Units, $ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Options Outstanding, Shares Available for Grant    
Shares Available for Grant, Beginning balance 1,945,025  
Shares Available for Grant, Awards authorized 2,974,638  
Shares Available for Grant, Granted (3,321,300)  
Shares Available for Grant, Exercised 0  
Shares Available for Grant, forfeited/cancelled 241,340  
Shares Available for Grant, Ending balance 1,839,703 1,945,025
Options Outstanding, Number of Shares Outstanding    
Number of Shares Outstanding, Beginning balance 10,504,412  
Number of Shares Outstanding, Options granted 3,321,300  
Number of Shares Outstanding, Options exercised (322,318)  
Number of Shares Outstanding, Options forfeited/cancelled (241,340)  
Number of Shares Outstanding, Ending balance 13,262,054 10,504,412
Number of Shares Outstanding, Exercisable 7,173,953  
Number of Shares Outstanding, Vested and expected to vest 13,262,054  
Options Outstanding, Weighted-Average Exercise Price Per Share    
Weighted-Average Exercise Price Per Share, Beginning balance $ 2.86  
Weighted-Average Exercise Price Per Share, Options granted 1.85  
Weighted-Average Exercise Price Per Share, Options exercised 1.38  
Weighted-Average Exercise Price Per Share, Options forfeited/cancelled 3.27  
Weighted-Average Exercise Price Per Share, Ending balance 2.64 $ 2.86
Weighted-Average Exercise Price Per Share, Exercisable 3.09  
Weighted-Average Exercise Price Per Share, Vested and expected to vest $ 2.64  
Options Outstanding, Weighted-Average Remaining Contractual Term (Years)    
Weighted-Average Remaining Contractual Term (Years) 7 years 10 months 27 days 7 years 9 months 18 days
Weighted-Average Remaining Contractual Term (Year), Exercisable 6 years 10 months 24 days  
Weighted-Average Remaining Contractual Term (Year), Vested and expected to vest 7 years 10 months 27 days  
Options Outstanding, Aggregate Intrinsic Value of Outstanding Options    
Aggregate Intrinsic Value of Outstanding Options $ 3 $ 545
Aggregate Intrinsic Value of Outstanding Options, Exercisable 3  
Aggregate Intrinsic Value of Outstanding Options, Vested and expected to vest $ 3