Net loss per share (Details) - shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Net loss per share
Potentially dilutive securities excluded from the computation of diluted weighted average shares outstanding	3,288,376	20,506,738
Redeemable convertible preferred shares
Net loss per share
Potentially dilutive securities excluded from the computation of diluted weighted average shares outstanding		17,550,802
Share options and unvested restricted share awards
Net loss per share
Potentially dilutive securities excluded from the computation of diluted weighted average shares outstanding	3,288,376	2,955,936