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Stock-Based Compensation - Summary of Stock-Based Compensation Activity (Detail) - USD ($)
$ / shares in Units, $ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Options Outstanding, Number of Shares Outstanding      
Number of Shares Outstanding, Beginning balance 7,470,601 7,470,601  
Number of Shares Outstanding, Ending balance 10,569,959    
2008 Plan and 2015 Plan [Member]      
Options Outstanding, Shares Available for Grant      
Shares Available for Grant, Beginning balance 1,503,770 1,503,770  
Shares Available for Grant, Awards authorized 2,095,808    
Shares Available for Grant, Granted (3,276,750)    
Shares Available for Grant, Exercised 0    
Shares Available for Grant, forfeited/cancelled 88,434    
Shares Available for Grant, Ending balance 411,262    
Options Outstanding, Number of Shares Outstanding      
Number of Shares Outstanding, Beginning balance 7,470,601 7,470,601  
Number of Shares Outstanding, Options granted 3,276,750    
Number of Shares Outstanding, Options exercised (88,958)    
Number of Shares Outstanding, Options forfeited/cancelled (88,434)    
Number of Shares Outstanding, Ending balance 10,569,959    
Number of Shares Outstanding, Exercisable 4,977,704    
Number of Shares Outstanding, Vested and expected to vest 10,569,959    
Options Outstanding, Weighted-Average Exercise Price Per Share      
Weighted-Average Exercise Price Per Share, Beginning balance $ 3.09 $ 3.09  
Weighted-Average Exercise Price Per Share, Options granted 2.38    
Weighted-Average Exercise Price Per Share, Options exercised 1.50    
Weighted-Average Exercise Price Per Share, Options forfeited/cancelled 4.70    
Weighted-Average Exercise Price Per Share, Ending balance 2.87    
Weighted-Average Exercise Price Per Share, Exercisable 3.03    
Weighted-Average Exercise Price Per Share, Vested and expected to vest $ 2.87    
Options Outstanding, Weighted-Average Remaining Contractual Term (Years)      
Weighted-Average Remaining Contractual Term (Years) 8 years 3 months 8 days 8 years 1 month 13 days  
Weighted-Average Remaining Contractual Term (Year), Exercisable 7 years 4 months 9 days    
Weighted-Average Remaining Contractual Term (Year), Vested and expected to vest 8 years 3 months 8 days    
Options Outstanding, Aggregate Intrinsic Value of Outstanding Options      
Aggregate Intrinsic Value of Outstanding Options $ 9,736   $ 12,363
Aggregate Intrinsic Value of Outstanding Options, Exercisable 5,520    
Aggregate Intrinsic Value of Outstanding Options, Vested and expected to vest $ 9,736