Summary of significant accounting policies - Net loss per share (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Potentially dilutive securities that have been excluded from the calculation of diluted net loss per share
Total shares of common stock equivalents	3,499,934	14,405,301	9,745,621
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		4,712,698
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	3,499,934	1,844,188	1,897,206