Collaboration agreements - Takeda agreements - Accounting (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Jan. 31, 2016
Accounting analysis
Collaboration revenue		$ 6,267	$ 3,262	$ 14,284	$ 13,175
Accounts receivable		731		731		$ 1,051
Deferred revenue, current		16,476		16,476		22,731
Net income (loss)		$ (7,732)	$ (5,837)	$ (24,740)	$ (14,959)	(13,700)
Net income (loss) per share attributable to common stockholders — basic and diluted		$ (0.35)	$ (4.56)	$ (2.94)	$ (11.72)
Takeda Agreements
Accounting analysis
Collaboration revenue		$ 5,768	$ 2,422	$ 12,052	$ 10,456
Accounts receivable		484		484		542
Deferred revenue		43,578		43,578		52,066
Deferred revenue, current		11,263		11,263		$ 16,536
Takeda Agreements | Change in estimate, Research service costs
Accounting analysis
Collaboration revenue		4,028		$ 4,028
Net income (loss)		$ 4,028
Net income (loss) per share attributable to common stockholders — basic and diluted		$ 0.18
XMT-1522 Agreement
Accounting analysis
Milestone payment upon achievement of IND Clearance Date							$ 20,000
Milestone payment received upon achievement of the IND Clearance Date	$ 20,000
Future technological improvements | Takeda Agreements
Accounting analysis
Estimated performance period in which revenue will be recognized				10 years
Joint research committee services | Takeda Agreements
Accounting analysis
Estimated performance period in which revenue will be recognized				6 years