Common stock - Shares reserved for future issuance (Details) - shares	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Stock-based compensation awards	2,031,528	1,189,314
Employee stock purchase plan	175,131	0
Total	4,408,921	11,379,012
Series A Preferred Stock
Class of Stock [Line Items]
Convertible Preferred Stock	0	3,683,637
Series B Preferred Stock
Class of Stock [Line Items]
Convertible Preferred Stock	0	3,287,471
Unvested restricted common stock
Class of Stock [Line Items]
Vesting of restricted common stock	2,202,262	3,218,590