Summary of significant accounting policies - Net loss per share (Details) - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share
Total shares of common stock equivalents	5,294,236	3,845,944
Options to purchase common stock
Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share
Total shares of common stock equivalents	4,794,236	3,845,944
Restricted stock units
Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share
Total shares of common stock equivalents	500,000