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Collaboration agreements - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Contract liabilities
Balance at Beginning of Period			$ 125,314	$ 147,653
Additions			5,860	1,710
Deductions			15,937	16,267
Balance at End of Period	$ 115,237	$ 133,096	115,237	133,096
Amounts included in the contract liability at the beginning of the period	$ 4,388	$ 4,503	$ 14,875	$ 17,963

X

- Definition

Change In Contract Assets And Liabilities

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

X

- Definition

Amount of decrease in obligation to transfer good or service to customer for which consideration has been received or is receivable.

+ References

No definition available.

+ Details

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Namespace Prefix:	mrsn_
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Balance Type:	debit
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X

- Definition

Amount of increase in obligation to transfer good or service to customer for which consideration has been received or is receivable.

+ References

No definition available.

+ Details

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

X

- Definition

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479837/606-10-45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-8

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479837/606-10-45-2

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Namespace Prefix:	us-gaap_
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Balance Type:	credit
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X

- Definition

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-8

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