Nature of business and basis of presentation - Termination of collaborative arrangement - (Details) - USD ($) $ in Thousands	3 Months Ended							6 Months Ended	9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018
Collaboration agreements
Net (loss) income	$ (16,792)	$ (17,071)	$ 21,901	$ (22,431)	$ (17,069)	$ (12,354)	$ (12,403)		$ (11,962)	$ (41,825)
Deferred revenue recognized	$ 834				$ 1,760				$ 41,550	$ 7,515
2016 Restated Tokeda Agreement and XMT-1522 Agreement
Collaboration agreements
Deferred revenue recognized			$ 39,965					$ 0