Summary of significant accounting policies - Revenue Recognition (Details)	12 Months Ended
Dec. 31, 2024
Accounting Policies [Abstract]
Billing term	30 days

Period over which entity receives payment upon billing.

No definition available.

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