Summary of significant accounting policies - Net loss per share (Details) - shares	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share
Total shares of common stock equivalents	3,755,959	15,790,086
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	3,755,959	2,761,794