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Share-Based Compensation - Summary of Activity Related to Stock Options (Details) - Stock Options
9 Months Ended
Sep. 30, 2019
$ / shares
shares
Dec. 31, 2018
$ / shares
shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Number of Shares, Options outstanding, Beginning balance | shares 3,463,096  
Number of Shares, Granted | shares 1,892,600  
Number of Shares, Exercised | shares (16,362)  
Number of Shares, Forfeited | shares (1,146,450)  
Number of Shares, Options outstanding, Ending balance | shares 4,192,884  
Number of Shares, Options exercisable | shares 2,204,649 1,583,749
Weighted Average Exercise Price, Options outstanding, Beginning balance | $ / shares $ 6.62  
Weighted Average Exercise Price, Granted | $ / shares 1.26  
Weighted Average Exercise Price, Exercised | $ / shares 0.40  
Weighted Average Exercise Price, Forfeited | $ / shares 5.00  
Weighted Average Exercise Price, Options outstanding, Ending balance | $ / shares 4.67  
Weighted Average Exercise Price, Options exercisable | $ / shares $ 5.61 $ 5.63