Restructuring - Narrative (Details)	Jul. 26, 2023
Restructuring and Related Activities [Abstract]
Percentage of positions to be eliminated	0.50

The number of positions to be eliminated in connection with a restructuring plan, as a percentage of total current positions.

No definition available.

Name:	mrsn_RestructuringAndRelatedCostNumberOfPositionsToBeEliminatedPercent
Namespace Prefix:	mrsn_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant