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Summary of Significant Accounting Policies - Revenue Recognition (Details) | 12 Months Ended
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Dec. 31, 2021
Accounting Policies [Abstract] |
Billing term | 30 days

X
- Definition Period over which entity receives payment upon billing. + References No definition available. + Details
Name: | mrsn_RevenueCollectionPeriod
Namespace Prefix: | mrsn_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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