Summary of significant accounting policies - Net loss per share (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Loss per Share
Warrants outstanding (in shares)	39,474	39,474
Anti-dilutive securities	6,869,189	5,207,582	3,856,932
Stock options
Net Loss per Share
Anti-dilutive securities	6,112,948	4,720,772	3,746,567
Unvested restricted stock units
Net Loss per Share
Anti-dilutive securities	716,767	447,336	0
Warrants
Net Loss per Share
Anti-dilutive securities	39,474	39,474	110,365
Exchange warrants
Net Loss per Share
Warrants outstanding (in shares)	2,575,000