Net loss per share - Anti-dilutive securities (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Common stock equivalents outstanding excluded from the computation of diluted net loss per share attributable to common stockholders
Outstanding anti-dilutive securities excluded from computation of diluted net loss per share	5,830	4,154
Stock options
Common stock equivalents outstanding excluded from the computation of diluted net loss per share attributable to common stockholders
Outstanding anti-dilutive securities excluded from computation of diluted net loss per share	4,798	3,959
Unvested restricted stock
Common stock equivalents outstanding excluded from the computation of diluted net loss per share attributable to common stockholders
Outstanding anti-dilutive securities excluded from computation of diluted net loss per share	1,032	195