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Common stock - Shares reserved for future issuance (Details) - shares
Dec. 31, 2017
Dec. 31, 2016
Class of Stock [Line Items]    
Stock-based compensation awards 3,439,820 2,722,573
Employee stock purchase plan 534,274 354,569
Total 4,149,082 4,273,934
Unvested restricted common stock    
Class of Stock [Line Items]    
Vesting of restricted common stock 174,988 1,196,792