Net loss per share - Anti-Diluted (Details) - shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive shares excluded from the calculation of diluted earnings per share, total	3,188,982	23,436,918
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive shares excluded from the calculation of diluted earnings per share, total	3,095,450	2,314,740
Unvested restricted common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive shares excluded from the calculation of diluted earnings per share, total	93,532	66,042
Redeemable convertible preferred stock (as converted to common stock)
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive shares excluded from the calculation of diluted earnings per share, total		21,056,136