Common stock - Shares reserved for future issuance (Details) - shares	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Stock-based compensation awards	2,722,573	2,031,528
Employee stock purchase plan	354,569	175,131
Total	4,273,934	4,408,921
Unvested restricted common stock
Class of Stock [Line Items]
Vesting of restricted common stock	1,196,792	2,202,262