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Stock-based compensation - Narrative (Details) - USD ($)
1 Months Ended 10 Months Ended 12 Months Ended 22 Months Ended
Nov. 30, 2015
Dec. 31, 2014
Dec. 31, 2015
Dec. 31, 2015
Sep. 30, 2014
Apr. 30, 2014
Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Stock-based compensation expense   $ 1,462,061 $ 6,597,359        
Shares granted   1,020,234 994,748        
Shares granted, weighted-average grant date fair value (in usd per share)   $ 2.20 $ 8.55        
Shares exercised   13,572 47,280        
Shares exercised, weighted average exercise price (in usd per share)   $ 0.60 $ 8.63        
Stock options exercised, intrinsic value   $ 51,719 $ 149,386        
Unrecognized compensation cost     $ 15,495,100 $ 15,495,100      
Unrecognized compensation cost, recognition period     2 years 4 months 13 days        
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       155,276      
Stock options, vesting period       4 years      
Stock options, contractual term       10 years      
Stock-based compensation expense   $ 149,000 $ 517,336        
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        
Performance Shares              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Shares granted 150,000            
Unrecognized compensation cost     $ 1,000,000 $ 1,000,000      
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
Shares remaining available for grant of stock awards     206,555 206,555