Collaboration agreements - 2016 Takeda and XMT-1522 Agreements - Accounting Analysis (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Collaboration agreements
Deferred revenue recognized	$ 834		$ 1,760		$ 41,550	$ 7,515
Collaboration revenue	844		$ 2,151		42,081	$ 9,405
Accounts receivable	298			$ 298	298		$ 459
Accounts payable	7,304			7,304	7,304		10,727
2016 Restated Tokeda Agreement and XMT-1522 Agreement
Collaboration agreements
Deferred revenue recognized		$ 39,965		0
Accounts receivable	26			26	26		9
Accounts payable	$ 2,754			$ 2,754	$ 2,754		$ 2,749