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Stock-Based Compensation - Additional Information (Details) - USD ($)
1 Months Ended 3 Months Ended 9 Months Ended
Jan. 31, 2019
Feb. 28, 2018
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Jan. 31, 2018
Share-based Compensation                        
Total grant date fair value of options vested     $ 557,000     $ 510,000     $ 1,800,000 $ 1,400,000    
Shares available for future issuance (in shares)     669,159           669,159   764,145  
Value of share issued     $ 28,000 $ 137,000 $ 13,000 1,332,000 $ 723,000 $ 309,000        
Weighted-average exercise price (per share)     $ 7.25           $ 7.25   $ 7.60  
Number of stock options granted (in shares)                 587,852      
Options remained outstanding     1,907,712           1,907,712   1,475,299  
Stock-based compensation expense     $ 895,000     $ 722,000     $ 2,397,000 $ 2,064,000    
Immediate vesting                        
Share-based Compensation                        
Incremental modification value   $ 63,000                    
Remaining vesting                        
Share-based Compensation                        
Incremental modification value   $ 537,000                    
Chief Executive Officer                        
Share-based Compensation                        
Number of vested and unvested stock option award modified   400,000                    
Weighted-average exercise price (per share)   $ 7.64                   $ 16.25
Incremental modification value   $ 600,000                    
Employees                        
Share-based Compensation                        
Weighted-average grant date fair value of options granted     $ 4.49     $ 4.48     $ 4.11 $ 4.56    
2016 Employee Stock Purchase Plan                        
Share-based Compensation                        
Increase in number of shares available for grant 170,955                      
Shares available for future issuance (in shares)     417,939           417,939      
Value of share issued                 $ 130,000 $ 137,000    
Number of shares issued (in shares)                 22,405 20,057