Summary of significant accounting policies - Net loss per share (Details) - shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share
Total shares of common stock equivalents	3,845,944	15,830,868
Series A convertible preferred stock
Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share
Total shares of common stock equivalents		612,446
Series B convertible preferred stock
Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share
Total shares of common stock equivalents		7,235,969
Series C convertible preferred stock
Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share
Total shares of common stock equivalents		5,179,877
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	3,845,944	2,802,576