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

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- Definition The number of positions to be eliminated in connection with a restructuring plan, as a percentage of total current positions. + References No definition available. + Details
Name: | mrsn_RestructuringAndRelatedCostNumberOfPositionsToBeEliminatedPercent
Namespace Prefix: | mrsn_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_RestructuringAndRelatedActivitiesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration