Net loss per share (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Net loss per share
Potentially dilutive securities excluded from the computation of diluted weighted average shares outstanding	2,584,666	19,845,847
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	2,584,666	2,295,045