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Stock-based compensation - Narrative (Details)
1 Months Ended 12 Months Ended
Jun. 14, 2018
employee
shares
Jun. 30, 2018
adviser
employee
shares
Dec. 31, 2018
USD ($)
employee
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock options, vesting period 48 months        
Stock-based compensation expense | $     $ 1,939,611 $ 4,219,165 $ 6,572,963
Shares granted (shares) 654,544   1,517,544 1,059,500 763,320
Shares granted, weighted-average grant date fair value (in usd per share) | $ / shares     $ 1.95 $ 2.80 $ 6.35
Shares exercised (shares)     97,310 1,576 8,516
Shares exercised, weighted average exercise price (in usd per share) | $ / shares     $ 1.21 $ 1.67 $ 2.59
Stock options exercised, intrinsic value | $     $ 349,974 $ 2,606 $ 64,302
Unrecognized compensation cost | $     $ 2,160,000    
Unrecognized compensation cost, recognition period     2 years 8 months 27 days    
Option awards held (shares)     2,320,981 2,580,491  
Number of employees granted options | employee 7        
Minimum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Post termination exercisability extended period 1 year        
Maximum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Post termination exercisability extended period 3 years        
Non-employee stock options          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Shares granted to non-employee consultants and members of the Scientific Advisory Board (shares)     30,000 0 14,670
Stock options, vesting period     4 years    
Stock options, contractual term     10 years    
Stock-based compensation expense | $     $ 124,000 $ 201,000 $ 370,000
Options to purchase common stock          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock options, contractual term     10 years    
Options to purchase common stock | Minimum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock options, vesting period     1 year    
Options to purchase common stock | Maximum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock options, vesting period     4 years    
2015 Plan          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Shares remaining available for grant of stock awards (shares)     1,740,416    
Employee stock option, one-year post termination exercisability          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Post termination exercisability period   3 months      
Option awards held (shares)   877,137      
Number of employees holding options | employee   6 6    
Number of advisers holding options | adviser   1      
Post termination exercisability extended period   1 year      
Number of employees terminated | employee     3    
Employee stock option, three-year post termination exercisability          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Post termination exercisability period   3 months      
Option awards held (shares)   500,000      
Number of employees holding options | employee   1      
Post termination exercisability extended period   3 years      
Selling, general and administrative expenses | Options to purchase common stock          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock-based compensation expense | $         $ 285,000