v3.19.2
Stock-Based Compensation - Summary of Stock Option Activity (Detail)
 6 Months Ended 12 Months Ended
Jun. 30, 2019
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]    
Shares, Outstanding at beginning balance | shares 3,351,132  
Shares, Granted | shares 4,622,428  
Shares, Expired/cancelled | shares (1,201,753)  
Shares, Outstanding at ending balance | shares 6,771,807 3,351,132
Shares, Expected to vest | shares 5,183,231  
Shares, Options exercisable | shares 1,588,576  
Weighted Average Exercise Price, Outstanding at beginning balance | $ / shares $ 3.73  
Weighted Average Exercise Price, Granted | $ / shares 1.86  
Weighted Average Exercise Price, Expired/cancelled | $ / shares 2.80  
Weighted Average Exercise Price, Outstanding at ending balance | $ / shares 2.62 $ 3.73
Weighted Average Exercise Price, Expected to vest | $ / shares 2.37  
Weighted Average Exercise Price, Options exercisable | $ / shares $ 3.43  
Weighted Average Remaining Contractual Term, Outstanding 8 years 7 months 24 days 8 years 2 months 26 days
Weighted Average Remaining Contractual Term, Expected to vest 9 years 4 months 6 days  
Weighted Average Remaining Contractual Term, Options exercisable 6 years 4 months 13 days  
Aggregate Intrinsic Value, Outstanding | $ $ 17,133 $ 142,788
Aggregate Intrinsic Value, Options exercisable | $ $ 17,133