Debt - Agreement (Details) - Credit Facility $ in Thousands		3 Months Ended
May 08, 2019 USD ($) installment	Sep. 30, 2019 USD ($)
Debt
Maximum borrowing capacity	$ 20,000
Number of principal advances \| installment	4
Drawn amount	$ 5,000	$ 5,000
Number of monthly payments of principal to be paid following period of monthly interest only payments \| installment	30
Final payment as percentage of original principal amount	5.00%
Commitment fee (as a percent)	3.00%
Liquidity ratio	1.50
Additional rate in event of default	5.00%
Debt issuance costs	$ 215
Prepayment occurs prior to May 8, 2020
Debt
Prepayment fee (as a percent)	3.00%
Prepayment occurs between May 8, 2020 and May 8, 2021
Debt
Prepayment fee (as a percent)	2.00%
Prepayment occurs after May 8, 2021 but before maturity date and first anniversary of funding date
Debt
Prepayment fee (as a percent)	1.00%
Minimum
Debt
Term loan tranche amount	$ 5,000
Interest rate (as a percent)	4.00%
Prime rate
Debt
Spread (as a percent)	(1.50%)

Represents the additional percentage of rate in event of default under debt instrument.

No definition available.

Name:	mrsn_DebtInstrumentAdditionalRateInEventOfDefault
Namespace Prefix:	mrsn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the minimum liquidity ratio requirement under the terms of the debt agreement.

+ References

No definition available.

+ Details

Name:	mrsn_DebtInstrumentLiquidityRatio
Namespace Prefix:	mrsn_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of consecutive equal monthly payments of principal to be paid following period of monthly interest only payments.

+ References

No definition available.

+ Details

Name:	mrsn_DebtInstrumentNumberOfConsecutiveEqualPaymentsOfPrincipal
Namespace Prefix:	mrsn_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of principal advances under debt instrument.

+ References

No definition available.

+ Details

Name:	mrsn_DebtInstrumentNumberOfPrincipalAdvances
Namespace Prefix:	mrsn_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the percentage of prepayment fee under debt instrument.

+ References

No definition available.

+ Details

Name:	mrsn_DebtInstrumentPrepaymentFeePercent
Namespace Prefix:	mrsn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the amount of each principal advances tranche under debt instrument.

+ References

No definition available.

+ Details

Name:	mrsn_DebtInstrumentPrincipalAdvancesEachTrancheAmount
Namespace Prefix:	mrsn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the final payment as percentage of original principal amount.

+ References

No definition available.

+ Details

Name:	mrsn_FinalPaymentAsPercentageOfOriginalPrincipalAmount
Namespace Prefix:	mrsn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28555-108399

+ Details

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

- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityCommitmentFeePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

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 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_DebtInstrumentAxis=mrsn_CreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=mrsn_PrepaymentOccursPriorToMay082020Member
Namespace Prefix:
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Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
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Balance Type:
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- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_VariableRateAxis=us-gaap_PrimeRateMember
Namespace Prefix:
Data Type:	na
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