Nature of business and basis of presentation - Termination of collaborative arrangement (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019
Collaboration agreements
Net (loss) income	$ (16,926)	$ (16,246)	$ (16,792)	$ (17,071)	$ 21,901
Deferred revenue recognized	11				40,592
Accumulated deficit	$ (209,300)	$ (192,374)
2016 Restated Tokeda Agreement and XMT-1522 Agreement
Collaboration agreements
Deferred revenue recognized					$ 39,965