Collaboration Agreements - Other Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Collaboration agreements
Collaboration revenue	$ 11	$ 41,035
Type of Revenue	mrsn:CollaborationMember	mrsn:CollaborationMember
Asana BioSciences
Collaboration agreements
Next potential milestone payment eligible to receive	$ 2,500
Asana BioSciences \| Limited services
Collaboration agreements
Collaboration revenue	0	$ 15
Asana BioSciences \| Milestone
Collaboration agreements
Collaboration revenue	$ 0	$ 0

Amount of the next potential milestone the company will be eligible to receive.

No definition available.

Name:	mrsn_CollaborationArrangementsMilestonePaymentReceivableNextMilestone
Namespace Prefix:	mrsn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

+ Details

Name:	us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Indicates product and service for revenue from satisfaction of performance obligation by transferring promised product and service to customer.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 55
-Paragraph 91
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118944142&loc=SL49130690-203046-203046

+ Details

Name:	us-gaap_RevenueFromContractWithCustomerProductAndServiceExtensibleList
Namespace Prefix:	us-gaap_
Data Type:	srt-types:extensibleListItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_TypeOfArrangementAxis=mrsn_CollaborationAndCommercialLicenseAgreementAsanaBioSciencesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=mrsn_LimitedServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=mrsn_ServiceMilestoneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: