Collaboration agreements - 2016 Takeda and XMT-1522 Agreements - Additional Accounting (Details) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) item $ / shares	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2018 USD ($) item $ / shares	Sep. 30, 2017 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jan. 01, 2018 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Research and development	$ 15,180	$ 11,412	$ 40,098	$ 32,145
Collaboration revenue	2,151	6,267	9,405	14,284
Accounts receivable	586		586		$ 784
Accounts payable	6,729		6,729		3,070
Net loss	$ (17,069)	$ (7,732)	$ (41,825)	$ (24,740)
Basic and diluted net income (loss) per share (in dollars per share) | $ / shares	$ (0.74)	$ (0.35)	$ (1.82)	$ (2.94)
Deferred revenue	$ 46,906		$ 46,906		52,439	$ 48,208
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		1
2016 Restated Tokeda Agreement and XMT-1522 Agreement
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Allocated Transaction Price	$ 86,123		$ 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
Adjusted transaction price	86,429		86,429
Collaboration revenue	82		4,697
2016 Restated Tokeda Agreement and XMT-1522 Agreement | Revised estimate of costs to complete research services
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Collaboration revenue	(1,380)		(1,380)
Net loss	$ (1,380)		$ (1,380)
Basic and diluted net income (loss) per share (in dollars per share) | $ / shares	$ (0.06)		$ (0.06)
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		$ 5,768		$ 12,052
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		$ 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		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		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		449
2014, 2015 and 2016 Takeda Agreements
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Accounts receivable	32		32		454
Accounts payable	530		530
Deferred revenue	41,104		41,104
XMT-1522 Agreement
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Research and development	2,210	924	5,013	1,719
ASC 808 costs billed to Takeda	1,165	$ 0	2,959	$ 0
Estimated amount of research and development fees received or expected to be received included in transaction price	$ 9,755		$ 9,755
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