Collaboration agreements - Takeda - Accounting Analysis (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Collaboration agreements
Deferred revenue recognized	$ 11	$ 834		$ 817	$ 41,550
Accounts payable	4,528			4,528		$ 7,296
Collaboration revenue	11	$ 844		817	$ 42,081
2016 Restated Tokeda Agreement and XMT-1522 Agreement
Collaboration agreements
Deferred revenue recognized			$ 39,965
Accounts payable	$ 2,335			$ 2,335		$ 2,335