Name:	mrsn_LeasesLineItems
Namespace Prefix:	mrsn_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Increase (decrease) in present value of lessee's discounted obligation for lease payments from operating lease due to lease modification

No definition available.

Name:	mrsn_LesseeOperatingLeaseModificationIncreaseDecreaseInOperatingLeaseLiability
Namespace Prefix:	mrsn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Amount of increase (decrease) in lessee's right to use underlying asset under operating lease due to lease modification

No definition available.

Name:	mrsn_LesseeOperatingLeaseModificationIncreaseDecreaseInRightOfUseAssets
Namespace Prefix:	mrsn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The amount of allowance granted and reimbursable by the landlord to a tenant to entice tenant to move into landlord's building which will enable the tenant to prepare the leased premises for tenants occupancy.

No definition available.

Name:	mrsn_TenantImprovementAllowance
Namespace Prefix:	mrsn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount, before accumulated amortization, of right-of-use asset from finance lease.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121603541&loc=SL77918627-209977

Name:	us-gaap_FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Indicates (true false) whether lessee has option to extend operating lease.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918673-209980

Name:	us-gaap_LesseeOperatingLeaseExistenceOfOptionToExtend
Namespace Prefix:	us-gaap_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918673-209980

Name:	us-gaap_LesseeOperatingLeaseRenewalTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

The amount of an asset, typically cash, provided to a counterparty to provide certain assurance of performance by the entity pursuant to the terms of a written or oral agreement, such as a lease.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8,17)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682