Collaboration agreements - 2016 Takeda and XMT-1522 Agreements - Accounting (Details)	3 Months Ended
Mar. 31, 2018 item
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Revenue, Practical Expedient, Initial Application and Transition, Nonrestatement of Modified Contract	true
2016 Restated Tokeda Agreement and XMT-1522 Agreement
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Number of arrangements	1

Indicates (true/false) whether practical expedient is elected under initial application of retrospective method to aggregate effect of contract modified before beginning of earliest period presented when identifying satisfied and unsatisfied performance obligation, determining transaction price and allocating transaction price to performance obligation.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 65
-Paragraph 1
-Subparagraph (f)(4)
-URI http://asc.fasb.org/extlink&oid=82856876&loc=SL49131195-203048

Name:	us-gaap_RevenuePracticalExpedientInitialApplicationAndTransitionNonrestatementOfModifiedContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration