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Stock-based compensation - Narrative (Details) - USD ($)
1 Months Ended 3 Months Ended 9 Months Ended
Aug. 31, 2016
May 31, 2016
Nov. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2014
Apr. 30, 2014
Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Unrecognized compensation cost       $ 14,469,895   $ 14,469,895        
Unrecognized compensation cost, recognition period (in years)           2 years 1 month 6 days        
Shares granted           759,820        
Stock-based compensation expense       $ 1,860,588 $ 1,506,679 $ 5,367,070 $ 4,892,998      
Employee stock option                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Stock options, contractual term           10 years        
Stock-based compensation expense $ 58,000 $ 227,000                
Performance Shares                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Shares granted     150,000              
Minimum | Employee stock option                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Stock options, vesting period (in years)           1 year        
Maximum | Employee stock option                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Stock options, vesting period (in years)           4 years        
Flex Pharma, Inc. 2014 Equity Incentive Plan                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Shares authorized               2,070,200 1,451,087 116,754
2015 Equity Incentive Plan                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Shares remaining available for grant of stock awards       402,326   402,326