Net loss per share (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Warrants | Series C Redeemable Convertible Preferred Stock Warrants (as converted)
Net loss per share
Potentially dilutive shares excluded in the computation of diluted net loss per share	70,833	70,833	70,833
Stock options outstanding
Net loss per share
Potentially dilutive shares excluded in the computation of diluted net loss per share	3,446,885	2,454,973	2,107,344
RSUs
Net loss per share
Potentially dilutive shares excluded in the computation of diluted net loss per share	75,314	85,000