Collaboration agreements - 2016 Takeda and XMT-1522 Agreements - Performance obligations (Details) - 2016 Restated Tokeda Agreement and XMT-1522 Agreement - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018
Performance obligations
Allocated Transaction Price	$ 86,123	$ 86,123
XMT-1522 license and research services
Performance obligations
Allocated Transaction Price	49,828
Joint research committee services for XMT-1522
Performance obligations
Allocated Transaction Price	449
Exclusive license to the first designated target and research services
Performance obligations
Allocated Transaction Price	6,611
Research license to the second designated target and research services
Performance obligations
Allocated Transaction Price	1,017
Material right related to the exclusive license to the second designated target
Performance obligations
Allocated Transaction Price	526
Exclusive license to the third designated target and research services
Performance obligations
Allocated Transaction Price	4,974
Exclusive license to the fourth designated target and research services
Performance obligations
Allocated Transaction Price	3,678
Material right related to the license to the fifth designated target and research services
Performance obligations
Allocated Transaction Price	3,506
Material right related to the license to the sixth designated target and research services
Performance obligations
Allocated Transaction Price	3,506
Material right to license to the seventh designated target and research services
Performance obligations
Allocated Transaction Price	3,506
Material right related to first replacement right for a designated target
Performance obligations
Allocated Transaction Price	3,506
Material right related to the second replacement right to a designated target
Performance obligations
Allocated Transaction Price	3,116
Rights to future technological improvements
Performance obligations
Allocated Transaction Price	1,750
Joint research committee services
Performance obligations
Allocated Transaction Price	$ 150