Collaboration agreements - 2016 Takeda and XMT-1522 Agreements - Additional Accounting (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) item	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Research and development	$ 12,256	$ 10,106
Accounts receivable	288		$ 784
Accounts payable	2,522		3,070
Deferred revenue	$ 50,462		52,439
License and related research and development services, including Phase 1 development and transfer certain materials and know how
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Number of performance obligations | item	1
2016 Restated Tokeda Agreement and XMT-1522 Agreement
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Allocated Transaction Price	$ 86,123			$ 86,123
Estimated amount of research and development fees received or expected to be received included in transaction price				14,023
Amount of milestone payments, which had not been received, included in the estimated transaction price				$ 0
Increase (decrease) in transaction price	0
Collaboration revenue	2,536
2016 Restated Tokeda Agreement and XMT-1522 Agreement | Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Collaboration revenue		3,530
2016 Restated Tokeda Agreement and XMT-1522 Agreement | Rights to future technological improvements
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Allocated Transaction Price	1,750
2016 Restated Tokeda Agreement and XMT-1522 Agreement | Joint research committee services
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Allocated Transaction Price	150
2016 Restated Tokeda Agreement and XMT-1522 Agreement | XMT-1522 license and research services
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Allocated Transaction Price	49,828
2016 Restated Tokeda Agreement and XMT-1522 Agreement | Joint research committee services for XMT-1522
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Allocated Transaction Price	449
2014, 2015 and 2016 Takeda Agreements
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Accounts receivable			454
Accounts payable	175
Deferred revenue	42,877
XMT-1522 Agreement
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Research and development	1,723	$ 294
ASC 808 costs billed to Takeda	$ 740
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01 | 2016 Restated Tokeda Agreement and XMT-1522 Agreement | XMT-1522 license and research services
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Expected recognition period	10 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01 | 2016 Restated Tokeda Agreement and XMT-1522 Agreement | Joint research committee services for XMT-1522
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Expected recognition period	6 years
Takeda
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Proceeds from IPO			$ 10,000