Summary of significant accounting policies - Net loss per share (Details) - shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share
Total shares of common stock equivalents	15,998,526	9,693,405
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	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	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	2,970,234	1,844,990