Significant accounting policies - Net loss per share (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share
Number of dilutive securities excluded in the calculation of diluted net loss per share	2,926,991	2,914,420	2,513,125
Unvested restricted common stock and restricted stock units
Outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share
Number of dilutive securities excluded in the calculation of diluted net loss per share	409,929	361,468	177,192
Outstanding stock options
Outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share
Number of dilutive securities excluded in the calculation of diluted net loss per share	2,507,062	2,476,911	2,249,843
Common stock
Outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share
Number of dilutive securities excluded in the calculation of diluted net loss per share	10,000	76,041	86,090
Obligation to issue warrants, in shares	93,341	93,341	93,341