Collaboration agreements - Takeda strategic R & D partnership (Details) - 2016 Restated Tokeda Agreement $ in Thousands		3 Months Ended
Jan. 02, 2019	Mar. 31, 2019 USD ($)	Jan. 31, 2016 item
Collaboration agreements
Number of targets \| item			7
Period of time until agreement terminated	45 days
Amount of costs billed \| $		$ 195

Period of time before which agreement will be terminated due to counter party serving prior written notice, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	mrsn_CollaborationArrangementsDurationToTermination
Namespace Prefix:	mrsn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration