Stockholders' Equity - Exchange Warrants (Details) - Exchange warrants - $ / shares	Mar. 02, 2020	Nov. 26, 2019
Warrants
Number of common shares exchanged for warrants		2,575,000
Number of shares of common stock into which warrants may be converted		2,575,000
Warrant exercise price per share (in dollars per share)		$ 0.0001
Shares issued upon exercise of warrants	2,574,971

Number of common shares exchanged for warrants.

No definition available.

Name:	mrsn_ClassOfWarrantOrRightSharesExchangedForWarrants
Namespace Prefix:	mrsn_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Shares of stock issued as a result of the exercise of warrants.

+ References

No definition available.

+ Details

Name:	mrsn_WarrantsExercisesInPeriod
Namespace Prefix:	mrsn_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Exercise price per share or per unit of warrants or rights outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i)(4))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of securities into which the class of warrant or right may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i)(2))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

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

- Details

Name:	us-gaap_StatementEquityComponentsAxis=mrsn_WarrantTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: