Net loss per share (Details) - shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Net loss per share
Potentially dilutive securities excluded from the computation of diluted weighted average shares outstanding	2,635,009	11,317,938
Redeemable convertible preferred shares
Net loss per share
Potentially dilutive securities excluded from the computation of diluted weighted average shares outstanding		9,433,910
Share options and unvested restricted share awards
Net loss per share
Potentially dilutive securities excluded from the computation of diluted weighted average shares outstanding	2,635,009	1,884,028