$MRSN
Mersana Therapeutics, Inc. Stock $mrsn

$mrsn Overview Financials Valuation SEC Filings Company Website

2024 Q1 Form 10-K Financial Statement

#000162828024007407 Filed on February 28, 2024

Income Statement

Concept	2024 Q1	2023 Q4	2023
Revenue	$9.245M	$10.70M	$36.86M
YoY Change	18.5%	-27.14%	38.65%
Cost Of Revenue
YoY Change
Gross Profit
YoY Change
Gross Profit Margin
Selling, General & Admin	$11.56M	$10.13M	$59.54M
YoY Change	-36.93%	-31.55%	4.53%
% of Gross Profit
Research & Development	$18.69M	$21.50M	$148.3M
YoY Change	-60.47%	-52.97%	-14.49%
% of Gross Profit
Depreciation & Amortization	$423.0K	$447.0K	$1.517M
YoY Change	32.6%	58.51%	63.65%
% of Gross Profit
Operating Expenses	$30.25M	$31.63M	$216.5M
YoY Change	-53.9%	-47.73%	-6.0%
Operating Profit
YoY Change
Interest Expense	$1.002M	$1.883M	$4.073M
YoY Change	1.93%	108.76%	22.39%
% of Operating Profit
Other Income/Expense, Net	$1.695M		$8.000M
YoY Change	3.48%		-1897.75%
Pretax Income	-$19.31M	-$19.54M	-$171.7M
YoY Change	-65.63%	-56.5%	-15.94%
Income Tax			$0.00
% Of Pretax Income
Net Earnings	-$19.30M	-$19.54M	-$171.7M
YoY Change	-65.66%	-56.5%	-15.92%
Net Earnings / Revenue	-208.76%	-182.64%	-465.88%
Basic Earnings Per Share	-$0.16		-$1.48
Diluted Earnings Per Share	-$0.16	-$0.16	-$1.48
COMMON SHARES
Basic Shares Outstanding	121.3M shares	120.6M shares	116.1M shares
Diluted Shares Outstanding	121.4M shares		116.1M shares

Balance Sheet

Concept	2024 Q1	2023 Q4	2023
SHORT-TERM ASSETS
Cash & Short-Term Investments	$183.1M	$209.1M	$209.1M
YoY Change	-33.14%	-25.52%	-25.52%
Cash & Equivalents	$75.19M	$174.6M	$174.6M
Short-Term Investments	$108.0M	$34.52M	$34.52M
Other Short-Term Assets	$4.392M	$4.973M	$4.973M
YoY Change	-44.32%	-41.54%	-41.54%
Inventory
Prepaid Expenses
Receivables		$0.00	$0.00
Other Receivables		$0.00	$0.00
Total Short-Term Assets	$187.5M	$214.1M	$214.1M
YoY Change	-33.45%	-32.94%	-32.94%
LONG-TERM ASSETS
Property, Plant & Equipment	$10.36M	$3.831M	$11.53M
YoY Change	-24.95%	-3.86%	-20.3%
Goodwill
YoY Change
Intangibles
YoY Change
Long-Term Investments
YoY Change
Other Assets	$478.0K	$478.0K	$478.0K
YoY Change	-18.71%	-27.69%	-27.69%
Total Long-Term Assets	$10.84M	$12.00M	$12.00M
YoY Change	-24.69%	-20.62%	-20.62%
TOTAL ASSETS
Total Short-Term Assets	$187.5M	$214.1M	$214.1M
Total Long-Term Assets	$10.84M	$12.00M	$12.00M
Total Assets	$198.4M	$226.1M	$226.1M
YoY Change	-33.03%	-32.39%	-32.39%
SHORT-TERM LIABILITIES
YoY Change
Accounts Payable	$3.086M	$7.319M	$7.319M
YoY Change	-83.0%	-47.54%	-47.54%
Accrued Expenses	$15.86M	$21.90M	$25.15M
YoY Change	-51.34%	-49.29%	-45.3%
Deferred Revenue	$25.62M	$28.15M
YoY Change	-10.95%	-8.05%
Short-Term Debt	$0.00	$2.083M	$0.00
YoY Change
Long-Term Debt Due	$5.208M	$2.083M	$2.227M
YoY Change			827.92%
Total Short-Term Liabilities	$53.41M	$63.64M	$63.64M
YoY Change	-35.97%	-30.48%	-30.48%
LONG-TERM LIABILITIES
Long-Term Debt	$19.79M	$23.15M	$23.20M
YoY Change	-20.83%	-7.14%	-7.68%
Other Long-Term Liabilities	$638.0K	$55.00K	$102.3M
YoY Change	355.71%	-72.91%	-18.55%
Total Long-Term Liabilities	$20.43M	$23.20M	$125.5M
YoY Change	-18.74%	-7.68%	-16.74%
TOTAL LIABILITIES
Total Short-Term Liabilities	$53.41M	$63.64M	$63.64M
Total Long-Term Liabilities	$20.43M	$23.20M	$125.5M
Total Liabilities	$170.4M	$189.2M	$189.2M
YoY Change	-26.53%	-21.93%	-21.93%
SHAREHOLDERS EQUITY
Retained Earnings	-$845.7M	-$826.4M
YoY Change	18.96%	26.23%
Common Stock	$873.7M	$863.3M
YoY Change	12.72%	15.58%
Preferred Stock
YoY Change
Treasury Stock (at cost)
YoY Change
Treasury Stock Shares
Shareholders Equity	$27.99M	$36.90M	$36.90M
YoY Change
Total Liabilities & Shareholders Equity	$198.4M	$226.1M	$226.1M
YoY Change	-33.03%	-32.39%	-32.39%

Cashflow Statement

Concept	2024 Q1	2023 Q4	2023
OPERATING ACTIVITIES
Net Income	-$19.30M	-$19.54M	-$171.7M
YoY Change	-65.66%	-56.5%	-15.92%
Depreciation, Depletion And Amortization	$423.0K	$447.0K	$1.517M
YoY Change	32.6%	58.51%	63.65%
Cash From Operating Activities	-$32.66M	-$31.96M	-$168.9M
YoY Change	12.59%	-37.61%	242.12%
INVESTING ACTIVITIES
Capital Expenditures	$132.0K	$386.0K	$2.168M
YoY Change	-85.51%	-50.83%	-1.32%
Acquisitions
YoY Change
Other Investing Activities	-$72.42M	$20.53M	$122.1M
YoY Change	-3238.93%	-146.0%	-181.09%
Cash From Investing Activities	-$72.55M	$20.15M	$119.9M
YoY Change	-5296.78%	-144.35%	-178.5%
FINANCING ACTIVITIES
Cash Dividend Paid
YoY Change
Common Stock Issuance & Retirement, Net
YoY Change
Debt Paid & Issued, Net
YoY Change
Cash From Financing Activities	$5.829M	92.00K	$94.68M
YoY Change	-72.95%	-99.78%	-38.13%
NET CHANGE
Cash From Operating Activities	-$32.66M	-31.96M	-$168.9M
Cash From Investing Activities	-$72.55M	20.15M	$119.9M
Cash From Financing Activities	$5.829M	92.00K	$94.68M
Net Change In Cash	-$99.38M	-11.72M	$45.68M
YoY Change	1539.85%	-78.76%	-193.1%
FREE CASH FLOW
Cash From Operating Activities	-$32.66M	-$31.96M	-$168.9M
Capital Expenditures	$132.0K	$386.0K	$2.168M
Free Cash Flow	-$32.79M	-$32.35M	-$171.1M
YoY Change	9.6%	-37.81%	231.75%

Facts In Submission

Frame	Concept Type	Concept / XBRL Key	Value	Unit
CY2022	us-gaap	Net Income Loss Available To Common Stockholders Diluted NetIncomeLossAvailableToCommonStockholdersDiluted	-204212000	usd
CY2022	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-2.18
CY2022	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-2.18
CY2021	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-2.41
CY2021	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-2.41
CY2022	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	93654243	shares
CY2022	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	93654243	shares
CY2022	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	247519000	usd
CY2023	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	2168000	usd
CY2022	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	2197000	usd
CY2023	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	119883000	usd
CY2022	us-gaap	Other Comprehensive Income Availableforsale Securities Adjustment Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent	-152000	usd
CY2022	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	-204364000	usd
CY2021	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	-170060000	usd
CY2023	dei	Entity Central Index Key EntityCentralIndexKey	0001442836
CY2023	dei	Document Fiscal Year Focus DocumentFiscalYearFocus	2023
CY2023	dei	Document Fiscal Period Focus DocumentFiscalPeriodFocus	FY
CY2023	dei	Amendment Flag AmendmentFlag	false
CY2023Q4	us-gaap	Finance Lease Liability Current Statement Of Financial Position Extensible List FinanceLeaseLiabilityCurrentStatementOfFinancialPositionExtensibleList	http://fasb.org/us-gaap/2023#OtherLiabilitiesCurrent
CY2022Q4	us-gaap	Finance Lease Liability Current Statement Of Financial Position Extensible List FinanceLeaseLiabilityCurrentStatementOfFinancialPositionExtensibleList	http://fasb.org/us-gaap/2023#OtherLiabilitiesCurrent
CY2022Q4	us-gaap	Finance Lease Liability Noncurrent Statement Of Financial Position Extensible List FinanceLeaseLiabilityNoncurrentStatementOfFinancialPositionExtensibleList	http://fasb.org/us-gaap/2023#OtherLiabilitiesNoncurrent
CY2023Q4	us-gaap	Finance Lease Liability Noncurrent Statement Of Financial Position Extensible List FinanceLeaseLiabilityNoncurrentStatementOfFinancialPositionExtensibleList	http://fasb.org/us-gaap/2023#OtherLiabilitiesNoncurrent
CY2023	dei	Document Annual Report DocumentAnnualReport	true
CY2023	dei	Document Period End Date DocumentPeriodEndDate	2023-12-31
CY2023	dei	Current Fiscal Year End Date CurrentFiscalYearEndDate	--12-31
CY2023	dei	Document Transition Report DocumentTransitionReport	false
CY2023	dei	Entity File Number EntityFileNumber	001-38129
CY2023	dei	Entity Registrant Name EntityRegistrantName	Mersana Therapeutics, Inc.
CY2023	dei	Entity Incorporation State Country Code EntityIncorporationStateCountryCode	DE
CY2023	dei	Entity Tax Identification Number EntityTaxIdentificationNumber	04-3562403
CY2023	dei	Entity Address Address Line1 EntityAddressAddressLine1	840 Memorial Drive
CY2023	dei	Entity Address City Or Town EntityAddressCityOrTown	Cambridge
CY2023	dei	Entity Address State Or Province EntityAddressStateOrProvince	MA
CY2023	dei	Entity Address Postal Zip Code EntityAddressPostalZipCode	02139
CY2023	dei	City Area Code CityAreaCode	617
CY2023	dei	Local Phone Number LocalPhoneNumber	498-0020
CY2023	dei	Security12b Title Security12bTitle	Common Stock, $0.0001 par value
CY2023	dei	Trading Symbol TradingSymbol	MRSN
CY2023	dei	Security Exchange Name SecurityExchangeName	NASDAQ
CY2023	dei	Entity Well Known Seasoned Issuer EntityWellKnownSeasonedIssuer	No
CY2023	dei	Entity Voluntary Filers EntityVoluntaryFilers	No
CY2023	dei	Entity Current Reporting Status EntityCurrentReportingStatus	Yes
CY2023	dei	Entity Interactive Data Current EntityInteractiveDataCurrent	Yes
CY2023	dei	Entity Filer Category EntityFilerCategory	Non-accelerated Filer
CY2023	dei	Entity Small Business EntitySmallBusiness	true
CY2023	dei	Entity Emerging Growth Company EntityEmergingGrowthCompany	false
CY2023	dei	Icfr Auditor Attestation Flag IcfrAuditorAttestationFlag	false
CY2023	dei	Document Fin Stmt Error Correction Flag DocumentFinStmtErrorCorrectionFlag	false
CY2023	dei	Entity Shell Company EntityShellCompany	false
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CY2024Q1	dei	Entity Common Stock Shares Outstanding EntityCommonStockSharesOutstanding	121303007	shares
CY2023	dei	Auditor Firm AuditorFirmId	42
CY2023	dei	Auditor Name AuditorName	Ernst & Young LLP
CY2023	dei	Auditor Location AuditorLocation	Boston, Massachusetts
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CY2023Q4	us-gaap	Contract With Customer Liability Current ContractWithCustomerLiabilityCurrent	28147000	usd
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CY2022Q4	us-gaap	Operating Lease Liability Noncurrent OperatingLeaseLiabilityNoncurrent	8575000	usd
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CY2023Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	120711745	shares
CY2023Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	120711745	shares
CY2022Q4	us-gaap	Common Stock Shares Issued CommonStockSharesIssued	105144864	shares
CY2022Q4	us-gaap	Common Stock Shares Outstanding CommonStockSharesOutstanding	105144864	shares
CY2023Q4	us-gaap	Common Stock Value CommonStockValue	12000	usd
CY2022Q4	us-gaap	Common Stock Value CommonStockValue	11000	usd
CY2022Q4	us-gaap	Additional Paid In Capital Common Stock AdditionalPaidInCapitalCommonStock	746889000	usd
CY2023Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	11000	usd
CY2022Q4	us-gaap	Accumulated Other Comprehensive Income Loss Net Of Tax AccumulatedOtherComprehensiveIncomeLossNetOfTax	-152000	usd
CY2023Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	-826361000	usd
CY2022Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	-654691000	usd
CY2023Q4	us-gaap	Stockholders Equity StockholdersEquity	36904000	usd
CY2022Q4	us-gaap	Stockholders Equity StockholdersEquity	92057000	usd
CY2023Q4	us-gaap	Liabilities And Stockholders Equity LiabilitiesAndStockholdersEquity	226060000	usd
CY2022Q4	us-gaap	Liabilities And Stockholders Equity LiabilitiesAndStockholdersEquity	334340000	usd
CY2023	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	36855000	usd
CY2022	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	26581000	usd
CY2021	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	43000	usd
CY2023	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	148269000	usd
CY2022	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	173385000	usd
CY2021	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	132013000	usd
CY2023	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	59543000	usd
CY2022	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	56963000	usd
CY2021	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	36888000	usd
CY2022	us-gaap	Restructuring Charges RestructuringCharges	0	usd
CY2021	us-gaap	Restructuring Charges RestructuringCharges	0	usd
CY2023	us-gaap	Operating Expenses OperatingExpenses	216525000	usd
CY2022	us-gaap	Operating Expenses OperatingExpenses	230348000	usd
CY2021	us-gaap	Operating Expenses OperatingExpenses	168901000	usd
CY2023	us-gaap	Investment Income Interest InvestmentIncomeInterest	12073000	usd
CY2022	us-gaap	Investment Income Interest InvestmentIncomeInterest	2883000	usd
CY2021	us-gaap	Investment Income Interest InvestmentIncomeInterest	65000	usd
CY2023	us-gaap	Interest Expense InterestExpense	4073000	usd
CY2022	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	173385000	usd
CY2021	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	132013000	usd
CY2023	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	59543000	usd
CY2022	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	56963000	usd
CY2021	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	36888000	usd
CY2023	us-gaap	Restructuring Charges RestructuringCharges	8713000	usd
CY2022	us-gaap	Restructuring Charges RestructuringCharges	0	usd
CY2021	us-gaap	Restructuring Charges RestructuringCharges	0	usd
CY2023	us-gaap	Operating Expenses OperatingExpenses	216525000	usd
CY2022	us-gaap	Operating Expenses OperatingExpenses	230348000	usd
CY2021	us-gaap	Operating Expenses OperatingExpenses	168901000	usd
CY2023	us-gaap	Investment Income Interest InvestmentIncomeInterest	12073000	usd
CY2022	us-gaap	Investment Income Interest InvestmentIncomeInterest	2883000	usd
CY2021	us-gaap	Investment Income Interest InvestmentIncomeInterest	65000	usd
CY2023	us-gaap	Interest Expense InterestExpense	4073000	usd
CY2022	us-gaap	Interest Expense InterestExpense	3328000	usd
CY2021	us-gaap	Interest Expense InterestExpense	1267000	usd
CY2021	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	-1202000	usd
CY2023	us-gaap	Net Income Loss NetIncomeLoss	-171670000	usd
CY2022	us-gaap	Net Income Loss NetIncomeLoss	-204212000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	-170060000	usd
CY2021	us-gaap	Other Comprehensive Income Availableforsale Securities Adjustment Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent	0	usd
CY2022	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	-204212000	usd
CY2023	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	116112891	shares
CY2023	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	116112891	shares
CY2020Q4	us-gaap	Stockholders Equity StockholdersEquity	228087000	usd
CY2021	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	1837000	usd
CY2021	us-gaap	Adjustments Related To Tax Withholding For Share Based Compensation AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation	259000	usd
CY2021	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Employee Stock Purchase Program Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationEmployeeStockPurchaseProgramRequisiteServicePeriodRecognition	640000	usd
CY2021	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	18409000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	-170060000	usd
CY2021Q4	us-gaap	Stockholders Equity StockholdersEquity	121741000	usd
CY2022	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	1331000	usd
CY2023	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Employee Stock Purchase Program Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationEmployeeStockPurchaseProgramRequisiteServicePeriodRecognition	1121000	usd
CY2023	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	21136000	usd
CY2023	us-gaap	Net Income Loss NetIncomeLoss	-171670000	usd
CY2023Q4	us-gaap	Stockholders Equity StockholdersEquity	36904000	usd
CY2023	us-gaap	Profit Loss ProfitLoss	-171670000	usd
CY2021	us-gaap	Profit Loss ProfitLoss	-170060000	usd
CY2023	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	1517000	usd
CY2022	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	927000	usd
CY2021	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	855000	usd
CY2023	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	4569000	usd
CY2022	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	1462000	usd
CY2021	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	0	usd
CY2022	us-gaap	Interest Expense InterestExpense	3328000	usd
CY2021	us-gaap	Interest Expense InterestExpense	1267000	usd
CY2023	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	8000000	usd
CY2022	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	-445000	usd
CY2021	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	-1202000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	-170060000	usd
CY2023	us-gaap	Other Comprehensive Income Availableforsale Securities Adjustment Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent	163000	usd
CY2023	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	116112891	shares
CY2023	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	116112891	shares
CY2020Q4	us-gaap	Stockholders Equity StockholdersEquity	228087000	usd
CY2021	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	1837000	usd
CY2021	us-gaap	Adjustments Related To Tax Withholding For Share Based Compensation AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation	259000	usd
CY2021	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Employee Stock Purchase Program Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationEmployeeStockPurchaseProgramRequisiteServicePeriodRecognition	640000	usd
CY2021	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	18409000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	-170060000	usd
CY2021Q4	us-gaap	Stockholders Equity StockholdersEquity	121741000	usd
CY2022	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	1331000	usd
CY2022	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Employee Stock Purchase Program Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationEmployeeStockPurchaseProgramRequisiteServicePeriodRecognition	1065000	usd
CY2022	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	21522000	usd
CY2022	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-152000	usd
CY2022	us-gaap	Net Income Loss NetIncomeLoss	-204212000	usd
CY2022Q4	us-gaap	Stockholders Equity StockholdersEquity	92057000	usd
CY2023	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	427000	usd
CY2023	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Employee Stock Purchase Program Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationEmployeeStockPurchaseProgramRequisiteServicePeriodRecognition	1121000	usd
CY2023	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	21136000	usd
CY2023	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	163000	usd
CY2023	us-gaap	Net Income Loss NetIncomeLoss	-171670000	usd
CY2023Q4	us-gaap	Stockholders Equity StockholdersEquity	36904000	usd
CY2023	us-gaap	Profit Loss ProfitLoss	-171670000	usd
CY2022	us-gaap	Profit Loss ProfitLoss	-204212000	usd
CY2021	us-gaap	Profit Loss ProfitLoss	-170060000	usd
CY2023	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	1517000	usd
CY2022	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	927000	usd
CY2021	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	855000	usd
CY2023	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	4569000	usd
CY2022	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	1462000	usd
CY2021	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	0	usd
CY2023	us-gaap	Share Based Compensation ShareBasedCompensation	21136000	usd
CY2022	us-gaap	Share Based Compensation ShareBasedCompensation	21522000	usd
CY2021	us-gaap	Share Based Compensation ShareBasedCompensation	18409000	usd
CY2023	mrsn	Noncash Operating Lease Expense NoncashOperatingLeaseExpense	2780000	usd
CY2022	mrsn	Noncash Operating Lease Expense NoncashOperatingLeaseExpense	2777000	usd
CY2021	mrsn	Noncash Operating Lease Expense NoncashOperatingLeaseExpense	1829000	usd
CY2023	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-687000	usd
CY2022	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-763000	usd
CY2021	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-723000	usd
CY2023	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	-30000000	usd
CY2022	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	30000000	usd
CY2021	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	0	usd
CY2023	us-gaap	Increase Decrease In Prepaid Deferred Expense And Other Assets IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets	-3534000	usd
CY2022	us-gaap	Increase Decrease In Prepaid Deferred Expense And Other Assets IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets	-3863000	usd
CY2023	us-gaap	Share Based Compensation ShareBasedCompensation	21136000	usd
CY2022	us-gaap	Share Based Compensation ShareBasedCompensation	21522000	usd
CY2021	us-gaap	Share Based Compensation ShareBasedCompensation	18409000	usd
CY2023	mrsn	Noncash Operating Lease Expense NoncashOperatingLeaseExpense	2780000	usd
CY2022	mrsn	Noncash Operating Lease Expense NoncashOperatingLeaseExpense	2777000	usd
CY2021	mrsn	Noncash Operating Lease Expense NoncashOperatingLeaseExpense	1829000	usd
CY2023	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-687000	usd
CY2022	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-763000	usd
CY2021	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-723000	usd
CY2023	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	-30000000	usd
CY2022	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	30000000	usd
CY2021	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	0	usd
CY2023	us-gaap	Increase Decrease In Prepaid Deferred Expense And Other Assets IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets	-3534000	usd
CY2022	us-gaap	Increase Decrease In Prepaid Deferred Expense And Other Assets IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets	-3863000	usd
CY2021	us-gaap	Increase Decrease In Prepaid Deferred Expense And Other Assets IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets	2734000	usd
CY2023	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	0	usd
CY2022	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	0	usd
CY2021	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	718000	usd
CY2023	us-gaap	Increase Decrease In Accounts Payable IncreaseDecreaseInAccountsPayable	-6080000	usd
CY2022	us-gaap	Increase Decrease In Accounts Payable IncreaseDecreaseInAccountsPayable	947000	usd
CY2021	us-gaap	Increase Decrease In Accounts Payable IncreaseDecreaseInAccountsPayable	483000	usd
CY2023	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	-20935000	usd
CY2022	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	13594000	usd
CY2023	us-gaap	Increase Decrease In Operating Lease Liability IncreaseDecreaseInOperatingLeaseLiability	-3143000	usd
CY2022	us-gaap	Increase Decrease In Operating Lease Liability IncreaseDecreaseInOperatingLeaseLiability	-2539000	usd
CY2021	us-gaap	Increase Decrease In Operating Lease Liability IncreaseDecreaseInOperatingLeaseLiability	-1827000	usd
CY2023	us-gaap	Increase Decrease In Contract With Customer Liability IncreaseDecreaseInContractWithCustomerLiability	-22139000	usd
CY2022	us-gaap	Increase Decrease In Contract With Customer Liability IncreaseDecreaseInContractWithCustomerLiability	143709000	usd
CY2021	us-gaap	Increase Decrease In Contract With Customer Liability IncreaseDecreaseInContractWithCustomerLiability	-43000	usd
CY2023	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	0	usd
CY2022	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	748000	usd
CY2021	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	525000	usd
CY2022	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	-49363000	usd
CY2021	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	-139988000	usd
CY2023	us-gaap	Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities	277970000	usd
CY2022	us-gaap	Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities	97000000	usd
CY2021	us-gaap	Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities	0	usd
CY2023	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	155919000	usd
CY2021	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	0	usd
CY2022	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	1331000	usd
CY2021	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	1837000	usd
CY2023	us-gaap	Proceeds From Issuance Of Shares Under Incentive And Share Based Compensation Plans ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlans	1121000	usd
CY2022	us-gaap	Proceeds From Issuance Of Shares Under Incentive And Share Based Compensation Plans ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlans	1065000	usd
CY2021	us-gaap	Proceeds From Issuance Of Shares Under Incentive And Share Based Compensation Plans ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlans	640000	usd
CY2023	us-gaap	Payments Related To Tax Withholding For Share Based Compensation PaymentsRelatedToTaxWithholdingForShareBasedCompensation	0	usd
CY2022	us-gaap	Payments Related To Tax Withholding For Share Based Compensation PaymentsRelatedToTaxWithholdingForShareBasedCompensation	0	usd
CY2021	us-gaap	Payments Related To Tax Withholding For Share Based Compensation PaymentsRelatedToTaxWithholdingForShareBasedCompensation	259000	usd
CY2023	mrsn	Proceeds From Debt Net Issuance Costs ProceedsFromDebtNetIssuanceCosts	-150000	usd
CY2022	mrsn	Proceeds From Debt Net Issuance Costs ProceedsFromDebtNetIssuanceCosts	0	usd
CY2021	mrsn	Proceeds From Debt Net Issuance Costs ProceedsFromDebtNetIssuanceCosts	24042000	usd
CY2021	us-gaap	Increase Decrease In Prepaid Deferred Expense And Other Assets IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets	2734000	usd
CY2023	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	0	usd
CY2022	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	0	usd
CY2021	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	718000	usd
CY2022	us-gaap	Increase Decrease In Accounts Payable IncreaseDecreaseInAccountsPayable	947000	usd
CY2021	us-gaap	Increase Decrease In Accounts Payable IncreaseDecreaseInAccountsPayable	483000	usd
CY2023	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	-20935000	usd
CY2022	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	13594000	usd
CY2021	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	12570000	usd
CY2023	us-gaap	Increase Decrease In Operating Lease Liability IncreaseDecreaseInOperatingLeaseLiability	-3143000	usd
CY2022	us-gaap	Increase Decrease In Operating Lease Liability IncreaseDecreaseInOperatingLeaseLiability	-2539000	usd
CY2021	us-gaap	Increase Decrease In Operating Lease Liability IncreaseDecreaseInOperatingLeaseLiability	-1827000	usd
CY2023	us-gaap	Increase Decrease In Contract With Customer Liability IncreaseDecreaseInContractWithCustomerLiability	-22139000	usd
CY2022	us-gaap	Increase Decrease In Contract With Customer Liability IncreaseDecreaseInContractWithCustomerLiability	143709000	usd
CY2021	us-gaap	Increase Decrease In Contract With Customer Liability IncreaseDecreaseInContractWithCustomerLiability	-43000	usd
CY2023	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	0	usd
CY2022	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	748000	usd
CY2021	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	525000	usd
CY2023	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	-168882000	usd
CY2022	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	-49363000	usd
CY2021	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	-139988000	usd
CY2023	us-gaap	Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities	277970000	usd
CY2022	us-gaap	Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities	97000000	usd
CY2023	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	155919000	usd
CY2022	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-152716000	usd
CY2021	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-648000	usd
CY2023	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	427000	usd
CY2022	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	1331000	usd
CY2021	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	1837000	usd
CY2023	us-gaap	Proceeds From Issuance Of Shares Under Incentive And Share Based Compensation Plans ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlans	1121000	usd
CY2022	us-gaap	Proceeds From Issuance Of Shares Under Incentive And Share Based Compensation Plans ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlans	1065000	usd
CY2021	us-gaap	Proceeds From Issuance Of Shares Under Incentive And Share Based Compensation Plans ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlans	640000	usd
CY2023	us-gaap	Payments Related To Tax Withholding For Share Based Compensation PaymentsRelatedToTaxWithholdingForShareBasedCompensation	0	usd
CY2022	us-gaap	Payments Related To Tax Withholding For Share Based Compensation PaymentsRelatedToTaxWithholdingForShareBasedCompensation	0	usd
CY2021	us-gaap	Payments Related To Tax Withholding For Share Based Compensation PaymentsRelatedToTaxWithholdingForShareBasedCompensation	259000	usd
CY2023	mrsn	Proceeds From Debt Net Issuance Costs ProceedsFromDebtNetIssuanceCosts	-150000	usd
CY2022	mrsn	Proceeds From Debt Net Issuance Costs ProceedsFromDebtNetIssuanceCosts	0	usd
CY2021	mrsn	Proceeds From Debt Net Issuance Costs ProceedsFromDebtNetIssuanceCosts	24042000	usd
CY2023	us-gaap	Repayments Of Debt RepaymentsOfDebt	0	usd
CY2022	us-gaap	Repayments Of Debt RepaymentsOfDebt	0	usd
CY2021	us-gaap	Repayments Of Debt RepaymentsOfDebt	5486000	usd
CY2023	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	262000	usd
CY2022	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	272000	usd
CY2021	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	215000	usd
CY2023	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	94675000	usd
CY2022	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	153017000	usd
CY2021	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	63646000	usd
CY2023	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	45676000	usd
CY2022	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	-49062000	usd
CY2021	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	-76990000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	129363000	usd
CY2021Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	178425000	usd
CY2023	us-gaap	Repayments Of Debt RepaymentsOfDebt	0	usd
CY2022	us-gaap	Repayments Of Debt RepaymentsOfDebt	0	usd
CY2023	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	262000	usd
CY2022	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	272000	usd
CY2021	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	215000	usd
CY2022	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	153017000	usd
CY2021	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	63646000	usd
CY2023	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	45676000	usd
CY2022	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	-49062000	usd
CY2021	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	-76990000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	129363000	usd
CY2021Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	178425000	usd
CY2020Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	255415000	usd
CY2023Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	175039000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	129363000	usd
CY2021Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	178425000	usd
CY2023	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	132000	usd
CY2022	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	753000	usd
CY2021	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	0	usd
CY2023	us-gaap	Debt Issuance Costs Incurred During Noncash Or Partial Noncash Transaction DebtIssuanceCostsIncurredDuringNoncashOrPartialNoncashTransaction	0	usd
CY2022	us-gaap	Debt Issuance Costs Incurred During Noncash Or Partial Noncash Transaction DebtIssuanceCostsIncurredDuringNoncashOrPartialNoncashTransaction	150000	usd
CY2021	us-gaap	Debt Issuance Costs Incurred During Noncash Or Partial Noncash Transaction DebtIssuanceCostsIncurredDuringNoncashOrPartialNoncashTransaction	0	usd
CY2023	mrsn	Common Stock Issuance Costs CommonStockIssuanceCosts	0	usd
CY2022	mrsn	Common Stock Issuance Costs CommonStockIssuanceCosts	131000	usd
CY2021	mrsn	Common Stock Issuance Costs CommonStockIssuanceCosts	0	usd
CY2023	us-gaap	Interest Paid Net InterestPaidNet	3380000	usd
CY2022	us-gaap	Interest Paid Net InterestPaidNet	2463000	usd
CY2021	us-gaap	Interest Paid Net InterestPaidNet	429000	usd
CY2023	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	0	usd
CY2022	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	298000	usd
CY2021	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	3783000	usd
CY2023	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	0	usd
CY2022	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	0	usd
CY2021	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	609000	usd
CY2023Q4	mrsn	Number Of Additional Preclinical Candidates NumberOfAdditionalPreclinicalCandidates	2	candidate
CY2023	us-gaap	Net Income Loss NetIncomeLoss	-171700000	usd
CY2022	us-gaap	Net Income Loss NetIncomeLoss	-204200000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	-170100000	usd
CY2023Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	-826400000	usd
CY2020Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	255415000	usd
CY2023Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	175039000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	129363000	usd
CY2021Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	178425000	usd
CY2023	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	132000	usd
CY2022	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	753000	usd
CY2021	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	0	usd
CY2023	us-gaap	Debt Issuance Costs Incurred During Noncash Or Partial Noncash Transaction DebtIssuanceCostsIncurredDuringNoncashOrPartialNoncashTransaction	0	usd
CY2022	us-gaap	Debt Issuance Costs Incurred During Noncash Or Partial Noncash Transaction DebtIssuanceCostsIncurredDuringNoncashOrPartialNoncashTransaction	150000	usd
CY2023	mrsn	Common Stock Issuance Costs CommonStockIssuanceCosts	0	usd
CY2022	mrsn	Common Stock Issuance Costs CommonStockIssuanceCosts	131000	usd
CY2021	mrsn	Common Stock Issuance Costs CommonStockIssuanceCosts	0	usd
CY2023	us-gaap	Interest Paid Net InterestPaidNet	3380000	usd
CY2022	us-gaap	Interest Paid Net InterestPaidNet	2463000	usd
CY2021	us-gaap	Interest Paid Net InterestPaidNet	429000	usd
CY2023	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	0	usd
CY2022	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	298000	usd
CY2021	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	3783000	usd
CY2023	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	0	usd
CY2022	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	0	usd
CY2021	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	609000	usd
CY2023Q4	mrsn	Number Of Additional Preclinical Candidates NumberOfAdditionalPreclinicalCandidates	2	candidate
CY2023	us-gaap	Net Income Loss NetIncomeLoss	-171700000	usd
CY2022	us-gaap	Net Income Loss NetIncomeLoss	-204200000	usd
CY2021	us-gaap	Net Income Loss NetIncomeLoss	-170100000	usd
CY2023Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	-826400000	usd
CY2023	us-gaap	Use Of Estimates UseOfEstimates	<div style="margin-top:12pt;text-align:justify"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-style:italic;font-weight:700;line-height:120%">Use of Estimates</span></div><div style="margin-top:12pt;text-align:justify"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">The preparation of the Company's consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, equity, revenue, expenses and related disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenue and expenses during the reporting period. On an ongoing basis, the Company's management evaluates its estimates which include, but are not limited to, management's judgments with respect to the identification of performance obligations and standalone selling prices of those performance obligations within its revenue arrangements, accrued preclinical, manufacturing and clinical expenses, valuation of stock-based awards and income taxes. Actual results could differ from those estimates.</span></div>
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0
CY2023	us-gaap	Use Of Estimates UseOfEstimates	<div style="margin-top:12pt;text-align:justify"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-style:italic;font-weight:700;line-height:120%">Use of Estimates</span></div><div style="margin-top:12pt;text-align:justify"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">The preparation of the Company's consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, equity, revenue, expenses and related disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenue and expenses during the reporting period. On an ongoing basis, the Company's management evaluates its estimates which include, but are not limited to, management's judgments with respect to the identification of performance obligations and standalone selling prices of those performance obligations within its revenue arrangements, accrued preclinical, manufacturing and clinical expenses, valuation of stock-based awards and income taxes. Actual results could differ from those estimates.</span></div>
CY2023	mrsn	Revenue Collection Period RevenueCollectionPeriod	P30D
CY2023	mrsn	Revenue Collection Period RevenueCollectionPeriod	P30D
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0
CY2023	us-gaap	Asset Impairment Charges AssetImpairmentCharges	0	usd
CY2021	us-gaap	Asset Impairment Charges AssetImpairmentCharges	0	usd
CY2022	us-gaap	Asset Impairment Charges AssetImpairmentCharges	0	usd
CY2023	us-gaap	Asset Impairment Charges AssetImpairmentCharges	0	usd
CY2021	us-gaap	Asset Impairment Charges AssetImpairmentCharges	0	usd
CY2022	us-gaap	Asset Impairment Charges AssetImpairmentCharges	0	usd
CY2022	us-gaap	Contract With Customer Liability Revenue Recognized ContractWithCustomerLiabilityRevenueRecognized	43000	usd
CY2023	us-gaap	Concentration Risk Credit Risk ConcentrationRiskCreditRisk	<div style="margin-top:12pt;text-align:justify"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-style:italic;font-weight:700;line-height:120%">Concentration of Credit Risk and Off-balance Sheet Risk</span></div><div style="margin-top:12pt;text-align:justify"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">Financial instruments that potentially subject the Company to concentrations of credit risk primarily consist of cash equivalents and marketable securities. Under its investment policy, the Company limits amounts invested in such securities by credit rating, maturity, industry group, investment type and issuer, except for securities issued by the U.S. government. The Company does not believe that it is subject to any significant concentrations of credit risk from these financial instruments. The Company has no financial instruments with off-balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements.</span></div>
CY2023	us-gaap	Concentration Risk Credit Risk Financial Instrument Maximum Exposure ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure	0	usd
CY2022Q4	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	147653000	usd
CY2023	mrsn	Contract With Customer Liability Increase ContractWithCustomerLiabilityIncrease	5517000	usd
CY2023	mrsn	Contract With Customer Liability Decrease ContractWithCustomerLiabilityDecrease	27856000	usd
CY2023Q4	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	125314000	usd
CY2021Q4	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	3944000	usd
CY2022	mrsn	Contract With Customer Liability Increase ContractWithCustomerLiabilityIncrease	170000000	usd
CY2022	mrsn	Contract With Customer Liability Decrease ContractWithCustomerLiabilityDecrease	26291000	usd
CY2022Q4	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	147653000	usd
CY2023	us-gaap	Contract With Customer Liability Revenue Recognized ContractWithCustomerLiabilityRevenueRecognized	27870000	usd
CY2023	us-gaap	Concentration Risk Credit Risk ConcentrationRiskCreditRisk	<div style="margin-top:12pt;text-align:justify"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-style:italic;font-weight:700;line-height:120%">Concentration of Credit Risk and Off-balance Sheet Risk</span></div><div style="margin-top:12pt;text-align:justify"><span style="color:#000000;font-family:'Times New Roman',sans-serif;font-size:10pt;font-weight:400;line-height:120%">Financial instruments that potentially subject the Company to concentrations of credit risk primarily consist of cash equivalents and marketable securities. Under its investment policy, the Company limits amounts invested in such securities by credit rating, maturity, industry group, investment type and issuer, except for securities issued by the U.S. government. The Company does not believe that it is subject to any significant concentrations of credit risk from these financial instruments. The Company has no financial instruments with off-balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements.</span></div>
CY2023	us-gaap	Concentration Risk Credit Risk Financial Instrument Maximum Exposure ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure	0	usd
CY2022Q4	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	147653000	usd
CY2023	mrsn	Contract With Customer Liability Increase ContractWithCustomerLiabilityIncrease	5517000	usd
CY2023	mrsn	Contract With Customer Liability Decrease ContractWithCustomerLiabilityDecrease	27856000	usd
CY2022	mrsn	Contract With Customer Liability Increase ContractWithCustomerLiabilityIncrease	170000000	usd
CY2022	mrsn	Contract With Customer Liability Decrease ContractWithCustomerLiabilityDecrease	26291000	usd
CY2022Q4	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	147653000	usd
CY2023	us-gaap	Contract With Customer Liability Revenue Recognized ContractWithCustomerLiabilityRevenueRecognized	27870000	usd
CY2022	us-gaap	Contract With Customer Liability Revenue Recognized ContractWithCustomerLiabilityRevenueRecognized	43000	usd
CY2022Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	128885000	usd
CY2023Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	174561000	usd
CY2021Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	177947000	usd
CY2022Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	128885000	usd
CY2022Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2023Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2021Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2022Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	129363000	usd
CY2023Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	175039000	usd
CY2022Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	128885000	usd
CY2023Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	174561000	usd
CY2021Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	177947000	usd
CY2022Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	128885000	usd
CY2022Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2023Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2021Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2022Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	129363000	usd
CY2023Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	175039000	usd
CY2021Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	178425000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	129363000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Amortized Cost Excluding Accrued Interest After Allowance For Credit Loss DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss	34512000	usd
CY2023Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	13000	usd
CY2023Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	2000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Excluding Accrued Interest DebtSecuritiesAvailableForSaleExcludingAccruedInterest	34523000	usd
CY2022Q4	us-gaap	Debt Securities Available For Sale Amortized Cost Excluding Accrued Interest After Allowance For Credit Loss DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss	151980000	usd
CY2022Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	31000	usd
CY2022Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	184000	usd
CY2022Q4	us-gaap	Debt Securities Available For Sale Excluding Accrued Interest DebtSecuritiesAvailableForSaleExcludingAccruedInterest	151827000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Unrealized Loss Position Number Of Positions DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions	1	security
CY2023Q4	us-gaap	Debt Securities Available For Sale Unrealized Loss Position Accumulated Loss DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss	5000000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position12 Months Or Longer Number Of Positions DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions	0	security
CY2023Q4	us-gaap	Debt Securities Available For Sale Amortized Cost Allowance For Credit Loss Excluding Accrued Interest DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest	0	usd
CY2022Q4	us-gaap	Debt Securities Available For Sale Amortized Cost Allowance For Credit Loss Excluding Accrued Interest DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest	0	usd
CY2023Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Two LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo	12500000	usd
CY2023Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Three LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree	10417000	usd
CY2023Q4	us-gaap	Debt Instrument Carrying Amount DebtInstrumentCarryingAmount	25000000	usd
CY2023Q4	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	25000000	shares
CY2021Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	178425000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	129363000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Amortized Cost Excluding Accrued Interest After Allowance For Credit Loss DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss	34512000	usd
CY2023Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	13000	usd
CY2023Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	2000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Excluding Accrued Interest DebtSecuritiesAvailableForSaleExcludingAccruedInterest	34523000	usd
CY2022Q4	us-gaap	Debt Securities Available For Sale Amortized Cost Excluding Accrued Interest After Allowance For Credit Loss DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss	151980000	usd
CY2022Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	31000	usd
CY2022Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	184000	usd
CY2022Q4	us-gaap	Debt Securities Available For Sale Excluding Accrued Interest DebtSecuritiesAvailableForSaleExcludingAccruedInterest	151827000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Unrealized Loss Position Number Of Positions DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions	1	security
CY2023Q4	us-gaap	Debt Securities Available For Sale Unrealized Loss Position Accumulated Loss DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss	5000000	usd
CY2023Q4	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position12 Months Or Longer Number Of Positions DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions	0	security
CY2023Q4	us-gaap	Debt Securities Available For Sale Amortized Cost Allowance For Credit Loss Excluding Accrued Interest DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest	0	usd
CY2022Q4	us-gaap	Debt Securities Available For Sale Amortized Cost Allowance For Credit Loss Excluding Accrued Interest DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest	0	usd
CY2023Q4	us-gaap	Property Plant And Equipment And Finance Lease Right Of Use Asset Before Accumulated Depreciation And Amortization PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization	12901000	usd
CY2022Q4	us-gaap	Property Plant And Equipment And Finance Lease Right Of Use Asset Before Accumulated Depreciation And Amortization PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization	11727000	usd
CY2023Q4	us-gaap	Property Plant And Equipment And Finance Lease Right Of Use Asset Accumulated Depreciation And Amortization PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization	9070000	usd
CY2022Q4	us-gaap	Property Plant And Equipment And Finance Lease Right Of Use Asset Accumulated Depreciation And Amortization PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization	7742000	usd
CY2023Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	3831000	usd
CY2022Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	3985000	usd
CY2023	us-gaap	Depreciation Depreciation	1500000	usd
CY2022	us-gaap	Depreciation Depreciation	900000	usd
CY2021	us-gaap	Depreciation Depreciation	900000	usd
CY2023Q4	us-gaap	Long Term Debt Current LongTermDebtCurrent	2083000	usd
CY2023Q4	mrsn	Debt Instrument Carrying Amount Noncurrent DebtInstrumentCarryingAmountNoncurrent	22917000	usd
CY2023Q4	us-gaap	Debt Instrument Unamortized Discount DebtInstrumentUnamortizedDiscount	237000	usd
CY2023Q4	us-gaap	Long Term Debt Noncurrent LongTermDebtNoncurrent	23148000	usd
CY2023Q4	us-gaap	Property Plant And Equipment And Finance Lease Right Of Use Asset Before Accumulated Depreciation And Amortization PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization	12901000	usd
CY2022Q4	us-gaap	Property Plant And Equipment And Finance Lease Right Of Use Asset Before Accumulated Depreciation And Amortization PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization	11727000	usd
CY2023Q4	us-gaap	Property Plant And Equipment And Finance Lease Right Of Use Asset Accumulated Depreciation And Amortization PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization	9070000	usd
CY2022Q4	us-gaap	Property Plant And Equipment And Finance Lease Right Of Use Asset Accumulated Depreciation And Amortization PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization	7742000	usd
CY2022Q4	us-gaap	Property Plant And Equipment Net PropertyPlantAndEquipmentNet	3985000	usd
CY2022	us-gaap	Depreciation Depreciation	900000	usd
CY2021	us-gaap	Depreciation Depreciation	900000	usd
CY2023Q4	us-gaap	Employee Related Liabilities Current EmployeeRelatedLiabilitiesCurrent	8807000	usd
CY2022Q4	us-gaap	Employee Related Liabilities Current EmployeeRelatedLiabilitiesCurrent	11558000	usd
CY2023Q4	mrsn	Accrued Clinical Expenses Current AccruedClinicalExpensesCurrent	5063000	usd
CY2022Q4	mrsn	Accrued Clinical Expenses Current AccruedClinicalExpensesCurrent	14822000	usd
CY2023Q4	mrsn	Accrued Research And Non Clinical Expenses AccruedResearchAndNonClinicalExpenses	3090000	usd
CY2022Q4	mrsn	Accrued Research And Non Clinical Expenses AccruedResearchAndNonClinicalExpenses	2767000	usd
CY2023Q4	mrsn	Accrued Manufacturing Expenses Current AccruedManufacturingExpensesCurrent	2566000	usd
CY2023Q4	us-gaap	Restructuring Reserve Current RestructuringReserveCurrent	1047000	usd
CY2022Q4	us-gaap	Restructuring Reserve Current RestructuringReserveCurrent	0	usd
CY2023Q4	us-gaap	Accrued Professional Fees Current AccruedProfessionalFeesCurrent	936000	usd
CY2022Q4	us-gaap	Accrued Professional Fees Current AccruedProfessionalFeesCurrent	1865000	usd
CY2023Q4	us-gaap	Other Accrued Liabilities Current OtherAccruedLiabilitiesCurrent	389000	usd
CY2022Q4	us-gaap	Other Accrued Liabilities Current OtherAccruedLiabilitiesCurrent	636000	usd
CY2023Q4	us-gaap	Accrued Liabilities Current AccruedLiabilitiesCurrent	21898000	usd
CY2023Q4	us-gaap	Debt Instrument Carrying Amount DebtInstrumentCarryingAmount	25000000	usd
CY2023Q4	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2023Q4	us-gaap	Employee Related Liabilities Current EmployeeRelatedLiabilitiesCurrent	8807000	usd
CY2022Q4	us-gaap	Employee Related Liabilities Current EmployeeRelatedLiabilitiesCurrent	11558000	usd
CY2023Q4	mrsn	Accrued Clinical Expenses Current AccruedClinicalExpensesCurrent	5063000	usd
CY2022Q4	mrsn	Accrued Clinical Expenses Current AccruedClinicalExpensesCurrent	14822000	usd
CY2023Q4	mrsn	Accrued Research And Non Clinical Expenses AccruedResearchAndNonClinicalExpenses	3090000	usd
CY2022Q4	mrsn	Accrued Research And Non Clinical Expenses AccruedResearchAndNonClinicalExpenses	2767000	usd
CY2023Q4	mrsn	Accrued Manufacturing Expenses Current AccruedManufacturingExpensesCurrent	2566000	usd
CY2022Q4	mrsn	Accrued Manufacturing Expenses Current AccruedManufacturingExpensesCurrent	11536000	usd
CY2023Q4	us-gaap	Restructuring Reserve Current RestructuringReserveCurrent	1047000	usd
CY2022Q4	us-gaap	Restructuring Reserve Current RestructuringReserveCurrent	0	usd
CY2023Q4	us-gaap	Accrued Professional Fees Current AccruedProfessionalFeesCurrent	936000	usd
CY2022Q4	us-gaap	Accrued Professional Fees Current AccruedProfessionalFeesCurrent	1865000	usd
CY2023Q4	us-gaap	Other Accrued Liabilities Current OtherAccruedLiabilitiesCurrent	389000	usd
CY2022Q4	us-gaap	Other Accrued Liabilities Current OtherAccruedLiabilitiesCurrent	636000	usd
CY2023Q4	us-gaap	Accrued Liabilities Current AccruedLiabilitiesCurrent	21898000	usd
CY2022Q4	us-gaap	Accrued Liabilities Current AccruedLiabilitiesCurrent	43184000	usd
CY2023Q4	us-gaap	Debt Instrument Carrying Amount DebtInstrumentCarryingAmount	25000000	usd
CY2023Q4	us-gaap	Long Term Debt Current LongTermDebtCurrent	2083000	usd
CY2023Q4	mrsn	Debt Instrument Carrying Amount Noncurrent DebtInstrumentCarryingAmountNoncurrent	22917000	usd
CY2023Q4	us-gaap	Debt Instrument Unamortized Discount DebtInstrumentUnamortizedDiscount	237000	usd
CY2023Q4	us-gaap	Debt Instrument Unamortized Premium DebtInstrumentUnamortizedPremium	468000	usd
CY2023Q4	us-gaap	Long Term Debt Noncurrent LongTermDebtNoncurrent	23148000	usd
CY2023Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Next Twelve Months LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths	2083000	usd
CY2023Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Two LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo	12500000	usd
CY2023Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Three LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree	10417000	usd
CY2023Q4	us-gaap	Debt Instrument Carrying Amount DebtInstrumentCarryingAmount	25000000	usd
CY2023Q4	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	25000000	shares
CY2023Q4	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2013Q4	us-gaap	Class Of Warrant Or Right Exercise Price Of Warrants Or Rights1 ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1	0.05
CY2013	mrsn	Contractual Life Of Warrants ContractualLifeOfWarrants	P10Y
CY2023Q4	us-gaap	Class Of Warrant Or Right Number Of Securities Called By Warrants Or Rights ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights	0	shares
CY2013Q4	us-gaap	Class Of Warrant Or Right Exercise Price Of Warrants Or Rights1 ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1	0.05
CY2013	mrsn	Contractual Life Of Warrants ContractualLifeOfWarrants	P10Y
CY2023Q4	us-gaap	Class Of Warrant Or Right Number Of Securities Called By Warrants Or Rights ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights	0	shares
CY2022Q2	us-gaap	Common Stock Par Or Stated Value Per Share CommonStockParOrStatedValuePerShare	0.0001
CY2022Q2	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	175000000	shares
CY2022Q2	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	350000000	shares
CY2023Q4	us-gaap	Common Stock Capital Shares Reserved For Future Issuance CommonStockCapitalSharesReservedForFutureIssuance	14736953	shares
CY2022Q4	us-gaap	Common Stock Capital Shares Reserved For Future Issuance CommonStockCapitalSharesReservedForFutureIssuance	11944664	shares
CY2022Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	10051283	shares
CY2022Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	9.84
CY2022	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Remaining Contractual Term2 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2	P7Y2M12D
CY2022Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue	8197000	usd
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Gross ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross	4017763	shares
CY2023	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Grants In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice	4.76
CY2023	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	102596	shares
CY2023	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Exercises In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice	4.25
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Expirations In Period ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod	3063605	shares
CY2023	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Expirations In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice	12.00
CY2023Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	10902845	shares
CY2023Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	7.42
CY2023	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Remaining Contractual Term2 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2	P6Y9M18D
CY2023Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue	1786000	usd
CY2023Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber	6499540	shares
CY2023Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice	8.23
CY2023	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Exercisable Weighted Average Remaining Contractual Term1 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1	P5Y6M
CY2023Q4	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Exercisable Intrinsic Value1 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1	711000	usd
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	3.82
CY2022	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	3.97
CY2021	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	11.71
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	300000	usd
CY2022	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	1500000	usd
CY2021	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	4300000	usd
CY2023	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	400000	usd
CY2022	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	1300000	usd
CY2021	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	1800000	usd
CY2022Q2	us-gaap	Common Stock Par Or Stated Value Per Share CommonStockParOrStatedValuePerShare	0.0001
CY2022Q2	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	175000000	shares
CY2022Q2	us-gaap	Common Stock Shares Authorized CommonStockSharesAuthorized	350000000	shares
CY2023Q4	us-gaap	Common Stock Capital Shares Reserved For Future Issuance CommonStockCapitalSharesReservedForFutureIssuance	14736953	shares
CY2022Q4	us-gaap	Common Stock Capital Shares Reserved For Future Issuance CommonStockCapitalSharesReservedForFutureIssuance	11944664	shares
CY2022Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	10051283	shares
CY2022Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	9.84
CY2022	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Remaining Contractual Term2 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2	P7Y2M12D
CY2022Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue	8197000	usd
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Gross ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross	4017763	shares
CY2023	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Grants In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice	4.76
CY2023	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	102596	shares
CY2023	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Exercises In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice	4.25
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Expirations In Period ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod	3063605	shares
CY2023	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Expirations In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice	12.00
CY2023Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	10902845	shares
CY2023Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	7.42
CY2023	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Remaining Contractual Term2 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2	P6Y9M18D
CY2023Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue	1786000	usd
CY2023Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber	6499540	shares
CY2023Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice	8.23
CY2023	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Exercisable Weighted Average Remaining Contractual Term1 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1	P5Y6M
CY2023Q4	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Exercisable Intrinsic Value1 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1	711000	usd
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	3.82
CY2022	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	3.97
CY2021	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	11.71
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	300000	usd
CY2022	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	1500000	usd
CY2021	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	4300000	usd
CY2023	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	400000	usd
CY2022	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	1300000	usd
CY2021	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	1800000	usd
CY2023	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	21136000	usd
CY2022	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	21522000	usd
CY2021	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	18409000	usd
CY2023	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	21136000	usd
CY2021	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	18409000	usd
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Risk Free Interest Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate	0.038
CY2022	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Risk Free Interest Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate	0.021
CY2021	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Risk Free Interest Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate	0.009
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0
CY2022	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0
CY2021	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0
CY2023	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Fair Value Assumptions Expected Term1 SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1	P6Y18D
CY2022	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Fair Value Assumptions Expected Term1 SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1	P5Y11M26D
CY2021	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Fair Value Assumptions Expected Term1 SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1	P6Y21D
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Volatility Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate	1.03
CY2022	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Volatility Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate	0.88
CY2021	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Volatility Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate	0.82
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0
CY2023	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	21136000	usd
CY2022	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	21522000	usd
CY2021	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	18409000	usd
CY2023	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	21136000	usd
CY2022	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	21522000	usd
CY2021	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	18409000	usd
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Risk Free Interest Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate	0.038
CY2022	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Risk Free Interest Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate	0.021
CY2021	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Risk Free Interest Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate	0.009
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0
CY2022	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0
CY2021	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0
CY2023	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Fair Value Assumptions Expected Term1 SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1	P6Y18D
CY2022	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Fair Value Assumptions Expected Term1 SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1	P5Y11M26D
CY2021	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Fair Value Assumptions Expected Term1 SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1	P6Y21D
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Volatility Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate	1.03
CY2022	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Volatility Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate	0.88
CY2021	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Volatility Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate	0.82
CY2023	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0
CY2023	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	14736953	shares
CY2022	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	11944664	shares
CY2021	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	9199512	shares
CY2023Q4	us-gaap	Security Deposit SecurityDeposit	500000	usd
CY2022Q4	us-gaap	Security Deposit SecurityDeposit	500000	usd
CY2023	us-gaap	Operating Lease Cost OperatingLeaseCost	3838000	usd
CY2022	us-gaap	Operating Lease Cost OperatingLeaseCost	3793000	usd
CY2021	us-gaap	Operating Lease Cost OperatingLeaseCost	3502000	usd
CY2023	us-gaap	Finance Lease Right Of Use Asset Amortization FinanceLeaseRightOfUseAssetAmortization	185000	usd
CY2022	us-gaap	Finance Lease Right Of Use Asset Amortization FinanceLeaseRightOfUseAssetAmortization	194000	usd
CY2021	us-gaap	Finance Lease Right Of Use Asset Amortization FinanceLeaseRightOfUseAssetAmortization	169000	usd
CY2023	us-gaap	Finance Lease Interest Expense FinanceLeaseInterestExpense	16000	usd
CY2022	us-gaap	Finance Lease Interest Expense FinanceLeaseInterestExpense	28000	usd
CY2021	us-gaap	Finance Lease Interest Expense FinanceLeaseInterestExpense	28000	usd
CY2023	mrsn	Finance Lease Cost FinanceLeaseCost	201000	usd
CY2022	mrsn	Finance Lease Cost FinanceLeaseCost	222000	usd
CY2021	mrsn	Finance Lease Cost FinanceLeaseCost	197000	usd
CY2022Q4	us-gaap	Finance Lease Right Of Use Asset Accumulated Amortization FinanceLeaseRightOfUseAssetAccumulatedAmortization	539000	usd
CY2023Q4	us-gaap	Finance Lease Liability Current FinanceLeaseLiabilityCurrent	144000	usd
CY2022Q4	us-gaap	Finance Lease Liability Current FinanceLeaseLiabilityCurrent	240000	usd
CY2023Q4	us-gaap	Finance Lease Liability Noncurrent FinanceLeaseLiabilityNoncurrent	54000	usd
CY2022Q4	us-gaap	Finance Lease Liability Noncurrent FinanceLeaseLiabilityNoncurrent	203000	usd
CY2023Q4	us-gaap	Operating Lease Weighted Average Remaining Lease Term1 OperatingLeaseWeightedAverageRemainingLeaseTerm1	P2Y3M18D
CY2022Q4	us-gaap	Operating Lease Weighted Average Remaining Lease Term1 OperatingLeaseWeightedAverageRemainingLeaseTerm1	P3Y3M18D
CY2023Q4	us-gaap	Finance Lease Weighted Average Remaining Lease Term1 FinanceLeaseWeightedAverageRemainingLeaseTerm1	P1Y1M6D
CY2023	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	14736953	shares
CY2022	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	11944664	shares
CY2021	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	9199512	shares
CY2023Q4	us-gaap	Security Deposit SecurityDeposit	500000	usd
CY2022Q4	us-gaap	Security Deposit SecurityDeposit	500000	usd
CY2023	us-gaap	Operating Lease Cost OperatingLeaseCost	3838000	usd
CY2022	us-gaap	Operating Lease Cost OperatingLeaseCost	3793000	usd
CY2021	us-gaap	Operating Lease Cost OperatingLeaseCost	3502000	usd
CY2023	us-gaap	Finance Lease Right Of Use Asset Amortization FinanceLeaseRightOfUseAssetAmortization	185000	usd
CY2022	us-gaap	Finance Lease Right Of Use Asset Amortization FinanceLeaseRightOfUseAssetAmortization	194000	usd
CY2021	us-gaap	Finance Lease Right Of Use Asset Amortization FinanceLeaseRightOfUseAssetAmortization	169000	usd
CY2023	us-gaap	Finance Lease Interest Expense FinanceLeaseInterestExpense	16000	usd
CY2022	us-gaap	Finance Lease Interest Expense FinanceLeaseInterestExpense	28000	usd
CY2021	us-gaap	Finance Lease Interest Expense FinanceLeaseInterestExpense	28000	usd
CY2023	mrsn	Finance Lease Cost FinanceLeaseCost	201000	usd
CY2022	mrsn	Finance Lease Cost FinanceLeaseCost	222000	usd
CY2021	mrsn	Finance Lease Cost FinanceLeaseCost	197000	usd
CY2023Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	7694000	usd
CY2022Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	10475000	usd
CY2023Q4	us-gaap	Operating Lease Liability Current OperatingLeaseLiabilityCurrent	3252000	usd
CY2022Q4	us-gaap	Operating Lease Liability Current OperatingLeaseLiabilityCurrent	2798000	usd
CY2023Q4	us-gaap	Operating Lease Liability Noncurrent OperatingLeaseLiabilityNoncurrent	5149000	usd
CY2022Q4	us-gaap	Operating Lease Liability Noncurrent OperatingLeaseLiabilityNoncurrent	8575000	usd
CY2023Q4	us-gaap	Finance Lease Right Of Use Asset Before Accumulated Amortization FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization	1038000	usd
CY2022Q4	us-gaap	Finance Lease Right Of Use Asset Before Accumulated Amortization FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization	1038000	usd
CY2023Q4	us-gaap	Finance Lease Right Of Use Asset Accumulated Amortization FinanceLeaseRightOfUseAssetAccumulatedAmortization	723000	usd
CY2023Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	7694000	usd
CY2022Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	10475000	usd
CY2023Q4	us-gaap	Operating Lease Liability Current OperatingLeaseLiabilityCurrent	3252000	usd
CY2023Q4	us-gaap	Operating Lease Liability Noncurrent OperatingLeaseLiabilityNoncurrent	5149000	usd
CY2022Q4	us-gaap	Operating Lease Liability Noncurrent OperatingLeaseLiabilityNoncurrent	8575000	usd
CY2023Q4	us-gaap	Finance Lease Right Of Use Asset Before Accumulated Amortization FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization	1038000	usd
CY2022Q4	us-gaap	Finance Lease Right Of Use Asset Before Accumulated Amortization FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization	1038000	usd
CY2023Q4	us-gaap	Finance Lease Right Of Use Asset Accumulated Amortization FinanceLeaseRightOfUseAssetAccumulatedAmortization	723000	usd
CY2022Q4	us-gaap	Finance Lease Right Of Use Asset Accumulated Amortization FinanceLeaseRightOfUseAssetAccumulatedAmortization	539000	usd
CY2023Q4	us-gaap	Finance Lease Liability Current FinanceLeaseLiabilityCurrent	144000	usd
CY2022Q4	us-gaap	Finance Lease Liability Current FinanceLeaseLiabilityCurrent	240000	usd
CY2023Q4	us-gaap	Finance Lease Liability Noncurrent FinanceLeaseLiabilityNoncurrent	54000	usd
CY2022Q4	us-gaap	Finance Lease Liability Noncurrent FinanceLeaseLiabilityNoncurrent	203000	usd
CY2023Q4	us-gaap	Operating Lease Weighted Average Remaining Lease Term1 OperatingLeaseWeightedAverageRemainingLeaseTerm1	P2Y3M18D
CY2022Q4	us-gaap	Operating Lease Weighted Average Remaining Lease Term1 OperatingLeaseWeightedAverageRemainingLeaseTerm1	P3Y3M18D
CY2023Q4	us-gaap	Finance Lease Weighted Average Remaining Lease Term1 FinanceLeaseWeightedAverageRemainingLeaseTerm1	P1Y1M6D
CY2022Q4	us-gaap	Finance Lease Weighted Average Remaining Lease Term1 FinanceLeaseWeightedAverageRemainingLeaseTerm1	P2Y
CY2023Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.108
CY2022Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.108
CY2023Q4	us-gaap	Finance Lease Weighted Average Discount Rate Percent FinanceLeaseWeightedAverageDiscountRatePercent	0.044
CY2022Q4	us-gaap	Finance Lease Weighted Average Discount Rate Percent FinanceLeaseWeightedAverageDiscountRatePercent	0.054
CY2023	us-gaap	Operating Lease Payments OperatingLeasePayments	4029000	usd
CY2022	us-gaap	Operating Lease Payments OperatingLeasePayments	3865000	usd
CY2023	us-gaap	Finance Lease Interest Payment On Liability FinanceLeaseInterestPaymentOnLiability	16000	usd
CY2022	us-gaap	Finance Lease Interest Payment On Liability FinanceLeaseInterestPaymentOnLiability	28000	usd
CY2023	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	262000	usd
CY2022	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	272000	usd
CY2022Q4	us-gaap	Finance Lease Weighted Average Remaining Lease Term1 FinanceLeaseWeightedAverageRemainingLeaseTerm1	P2Y
CY2023Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.108
CY2022Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.108
CY2023Q4	us-gaap	Finance Lease Weighted Average Discount Rate Percent FinanceLeaseWeightedAverageDiscountRatePercent	0.044
CY2022Q4	us-gaap	Finance Lease Weighted Average Discount Rate Percent FinanceLeaseWeightedAverageDiscountRatePercent	0.054
CY2023	us-gaap	Operating Lease Payments OperatingLeasePayments	4029000	usd
CY2022	us-gaap	Operating Lease Payments OperatingLeasePayments	3865000	usd
CY2022	us-gaap	Finance Lease Interest Payment On Liability FinanceLeaseInterestPaymentOnLiability	28000	usd
CY2023	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	262000	usd
CY2022	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	272000	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due Next Twelve Months LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths	3975000	usd
CY2023Q4	us-gaap	Finance Lease Liability Payments Due Next Twelve Months FinanceLeaseLiabilityPaymentsDueNextTwelveMonths	141000	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Two LesseeOperatingLeaseLiabilityPaymentsDueYearTwo	4187000	usd
CY2023Q4	us-gaap	Finance Lease Liability Payments Due Year Two FinanceLeaseLiabilityPaymentsDueYearTwo	48000	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Three LesseeOperatingLeaseLiabilityPaymentsDueYearThree	1310000	usd
CY2023Q4	us-gaap	Finance Lease Liability Payments Due Year Three FinanceLeaseLiabilityPaymentsDueYearThree	8000	usd
CY2023Q4	mrsn	Lessee Operating Lease Liability To Be Paid Year Four And Thereafter LesseeOperatingLeaseLiabilityToBePaidYearFourAndThereafter	0	usd
CY2023Q4	mrsn	Finance Lease Liability To Be Paid Year Four And Thereafter FinanceLeaseLiabilityToBePaidYearFourAndThereafter	0	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due LesseeOperatingLeaseLiabilityPaymentsDue	9472000	usd
CY2023Q4	us-gaap	Finance Lease Liability Payments Due FinanceLeaseLiabilityPaymentsDue	197000	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Undiscounted Excess Amount LesseeOperatingLeaseLiabilityUndiscountedExcessAmount	1069000	usd
CY2023Q4	us-gaap	Finance Lease Liability Undiscounted Excess Amount FinanceLeaseLiabilityUndiscountedExcessAmount	0	usd
CY2023Q4	us-gaap	Operating Lease Liability OperatingLeaseLiability	8403000	usd
CY2023Q4	us-gaap	Finance Lease Liability FinanceLeaseLiability	197000	usd
CY2023	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	0	usd
CY2021	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	0	usd
CY2022	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	0	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due Next Twelve Months LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths	3975000	usd
CY2023Q4	us-gaap	Finance Lease Liability Payments Due Next Twelve Months FinanceLeaseLiabilityPaymentsDueNextTwelveMonths	141000	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Two LesseeOperatingLeaseLiabilityPaymentsDueYearTwo	4187000	usd
CY2023Q4	us-gaap	Finance Lease Liability Payments Due Year Two FinanceLeaseLiabilityPaymentsDueYearTwo	48000	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Three LesseeOperatingLeaseLiabilityPaymentsDueYearThree	1310000	usd
CY2023Q4	us-gaap	Finance Lease Liability Payments Due Year Three FinanceLeaseLiabilityPaymentsDueYearThree	8000	usd
CY2023Q4	mrsn	Lessee Operating Lease Liability To Be Paid Year Four And Thereafter LesseeOperatingLeaseLiabilityToBePaidYearFourAndThereafter	0	usd
CY2023Q4	mrsn	Finance Lease Liability To Be Paid Year Four And Thereafter FinanceLeaseLiabilityToBePaidYearFourAndThereafter	0	usd
CY2023Q4	us-gaap	Lessee Operating Lease Liability Payments Due LesseeOperatingLeaseLiabilityPaymentsDue	9472000	usd
CY2023Q4	us-gaap	Finance Lease Liability Payments Due FinanceLeaseLiabilityPaymentsDue	197000	usd
CY2023Q4	us-gaap	Operating Lease Liability OperatingLeaseLiability	8403000	usd
CY2023Q4	us-gaap	Finance Lease Liability FinanceLeaseLiability	197000	usd
CY2023	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	0	usd
CY2021	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	0	usd
CY2022	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	0	usd
CY2023	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.210
CY2022	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.210
CY2023	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.055
CY2022	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.057
CY2023	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense EffectiveIncomeTaxRateReconciliationNondeductibleExpense	-0.001
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense EffectiveIncomeTaxRateReconciliationNondeductibleExpense	-0.001
CY2023	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	-0.040
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	-0.044
CY2023	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Share Based Compensation Cost EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost	-0.019
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Share Based Compensation Cost EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost	-0.014
CY2023	us-gaap	Effective Income Tax Rate Reconciliation Change In Deferred Tax Assets Valuation Allowance EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance	-0.285
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Change In Deferred Tax Assets Valuation Allowance EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance	-0.296
CY2023	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0
CY2022	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0
CY2023	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.210
CY2022	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.210
CY2023	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.055
CY2022	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.057
CY2023	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense EffectiveIncomeTaxRateReconciliationNondeductibleExpense	-0.001
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense EffectiveIncomeTaxRateReconciliationNondeductibleExpense	-0.001
CY2023	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	-0.040
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	-0.044
CY2023	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Share Based Compensation Cost EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost	-0.019
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense Share Based Compensation Cost EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost	-0.014
CY2023	us-gaap	Effective Income Tax Rate Reconciliation Change In Deferred Tax Assets Valuation Allowance EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance	-0.285
CY2022	us-gaap	Effective Income Tax Rate Reconciliation Change In Deferred Tax Assets Valuation Allowance EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance	-0.296
CY2023	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0
CY2022	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0
CY2023Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	126798000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	113981000	usd
CY2023Q4	mrsn	Deferred Tax Assets Capitalized Research And Development Costs DeferredTaxAssetsCapitalizedResearchAndDevelopmentCosts	67107000	usd
CY2022Q4	mrsn	Deferred Tax Assets Capitalized Research And Development Costs DeferredTaxAssetsCapitalizedResearchAndDevelopmentCosts	40380000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Share Based Compensation Cost DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost	6899000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Share Based Compensation Cost DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost	6522000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Accrued Liabilities DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAccruedLiabilities	3142000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Accrued Liabilities DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAccruedLiabilities	3340000	usd
CY2023Q4	mrsn	Deferred Tax Assets Lease Liabilities DeferredTaxAssetsLeaseLiabilities	2274000	usd
CY2022Q4	mrsn	Deferred Tax Assets Lease Liabilities DeferredTaxAssetsLeaseLiabilities	3096000	usd
CY2023Q4	mrsn	Deferred Tax Assets Capitalized Licenses DeferredTaxAssetsCapitalizedLicenses	2725000	usd
CY2022Q4	mrsn	Deferred Tax Assets Capitalized Licenses DeferredTaxAssetsCapitalizedLicenses	3077000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Deferred Income DeferredTaxAssetsDeferredIncome	3688000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Deferred Income DeferredTaxAssetsDeferredIncome	1062000	usd
CY2023Q4	mrsn	Deferred Tax Assets Depreciation DeferredTaxAssetsDepreciation	353000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	156000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	116000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	241551000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	193440000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	239468000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	190588000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	126798000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	113981000	usd
CY2023Q4	mrsn	Deferred Tax Assets Capitalized Research And Development Costs DeferredTaxAssetsCapitalizedResearchAndDevelopmentCosts	67107000	usd
CY2022Q4	mrsn	Deferred Tax Assets Capitalized Research And Development Costs DeferredTaxAssetsCapitalizedResearchAndDevelopmentCosts	40380000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards DeferredTaxAssetsTaxCreditCarryforwards	28409000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards DeferredTaxAssetsTaxCreditCarryforwards	21429000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Share Based Compensation Cost DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost	6899000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Share Based Compensation Cost DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost	6522000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Accrued Liabilities DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAccruedLiabilities	3142000	usd
CY2023Q4	mrsn	Deferred Tax Assets Lease Liabilities DeferredTaxAssetsLeaseLiabilities	2274000	usd
CY2022Q4	mrsn	Deferred Tax Assets Lease Liabilities DeferredTaxAssetsLeaseLiabilities	3096000	usd
CY2022Q4	mrsn	Deferred Tax Assets Capitalized Licenses DeferredTaxAssetsCapitalizedLicenses	3077000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Deferred Income DeferredTaxAssetsDeferredIncome	3688000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Deferred Income DeferredTaxAssetsDeferredIncome	1062000	usd
CY2023Q4	mrsn	Deferred Tax Assets Depreciation DeferredTaxAssetsDepreciation	353000	usd
CY2022Q4	mrsn	Deferred Tax Assets Depreciation DeferredTaxAssetsDepreciation	437000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	156000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	116000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	241551000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	193440000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	239468000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	190588000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	2083000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	2852000	usd
CY2023Q4	us-gaap	Deferred Tax Liabilities Leasing Arrangements DeferredTaxLiabilitiesLeasingArrangements	2083000	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities Leasing Arrangements DeferredTaxLiabilitiesLeasingArrangements	2852000	usd
CY2023Q4	us-gaap	Deferred Income Tax Liabilities DeferredIncomeTaxLiabilities	2083000	usd
CY2022Q4	us-gaap	Deferred Income Tax Liabilities DeferredIncomeTaxLiabilities	2852000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Liabilities Net DeferredTaxAssetsLiabilitiesNet	0	usd
CY2022Q4	us-gaap	Deferred Tax Assets Liabilities Net DeferredTaxAssetsLiabilitiesNet	0	usd
CY2023Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	239500000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	190600000	usd
CY2023	us-gaap	Valuation Allowance Deferred Tax Asset Change In Amount ValuationAllowanceDeferredTaxAssetChangeInAmount	48900000	usd
CY2022Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	0	usd
CY2023Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	0	usd
CY2022Q4	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Accrued UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued	0	usd
CY2023Q4	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Accrued UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued	0	usd
CY2023Q4	mrsn	Income Taxes Number Of State Jurisdictions IncomeTaxesNumberOfStateJurisdictions	8	jurisdiction
CY2023Q4	mrsn	Income Taxes Number Of Audits In Progress IncomeTaxesNumberOfAuditsInProgress	0	audit
CY2023	us-gaap	Defined Contribution Plan Maximum Annual Contributions Per Employee Percent DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent	0.95
CY2021	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent Of Match DefinedContributionPlanEmployerMatchingContributionPercentOfMatch	1
CY2023	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent Of Match DefinedContributionPlanEmployerMatchingContributionPercentOfMatch	1
CY2023	mrsn	Defined Contribution Plan Eligible Age To Participate DefinedContributionPlanEligibleAgeToParticipate	P21Y
CY2023Q4	mrsn	Defined Contribution Plan Minimum Number Of Hours For Eligibility DefinedContributionPlanMinimumNumberOfHoursForEligibility	1000	hour
CY2023Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	2083000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	2852000	usd
CY2023Q4	us-gaap	Deferred Tax Liabilities Leasing Arrangements DeferredTaxLiabilitiesLeasingArrangements	2083000	usd
CY2022Q4	us-gaap	Deferred Tax Liabilities Leasing Arrangements DeferredTaxLiabilitiesLeasingArrangements	2852000	usd
CY2023Q4	us-gaap	Deferred Income Tax Liabilities DeferredIncomeTaxLiabilities	2083000	usd
CY2022Q4	us-gaap	Deferred Income Tax Liabilities DeferredIncomeTaxLiabilities	2852000	usd
CY2023Q4	us-gaap	Deferred Tax Assets Liabilities Net DeferredTaxAssetsLiabilitiesNet	0	usd
CY2022Q4	us-gaap	Deferred Tax Assets Liabilities Net DeferredTaxAssetsLiabilitiesNet	0	usd
CY2023Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	239500000	usd
CY2022Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	190600000	usd
CY2023	us-gaap	Valuation Allowance Deferred Tax Asset Change In Amount ValuationAllowanceDeferredTaxAssetChangeInAmount	48900000	usd
CY2022Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	0	usd
CY2023Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	0	usd
CY2022Q4	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Accrued UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued	0	usd
CY2023Q4	mrsn	Income Taxes Number Of State Jurisdictions IncomeTaxesNumberOfStateJurisdictions	8	jurisdiction
CY2023Q4	mrsn	Income Taxes Number Of Audits In Progress IncomeTaxesNumberOfAuditsInProgress	0	audit
CY2023	mrsn	Defined Contribution Plan Eligible Age To Participate DefinedContributionPlanEligibleAgeToParticipate	P21Y
CY2023Q4	mrsn	Defined Contribution Plan Minimum Number Of Hours For Eligibility DefinedContributionPlanMinimumNumberOfHoursForEligibility	1000	hour
CY2023	us-gaap	Defined Contribution Plan Maximum Annual Contributions Per Employee Percent DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent	0.95
CY2021	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent Of Match DefinedContributionPlanEmployerMatchingContributionPercentOfMatch	1
CY2023	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent Of Match DefinedContributionPlanEmployerMatchingContributionPercentOfMatch	1
CY2022	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent Of Match DefinedContributionPlanEmployerMatchingContributionPercentOfMatch	1
CY2022	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent DefinedContributionPlanEmployerMatchingContributionPercent	0.04
CY2023	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent DefinedContributionPlanEmployerMatchingContributionPercent	0.04
CY2021	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent DefinedContributionPlanEmployerMatchingContributionPercent	0.04
CY2023	us-gaap	Defined Contribution Plan Cost Recognized DefinedContributionPlanCostRecognized	1300000	usd
CY2022	us-gaap	Defined Contribution Plan Cost Recognized DefinedContributionPlanCostRecognized	1100000	usd
CY2021	us-gaap	Defined Contribution Plan Cost Recognized DefinedContributionPlanCostRecognized	800000	usd
CY2023Q3	mrsn	Restructuring And Related Cost Number Of Positions To Be Eliminated Percent RestructuringAndRelatedCostNumberOfPositionsToBeEliminatedPercent	0.50
CY2023Q4	us-gaap	Restructuring And Related Cost Cost Incurred To Date1 RestructuringAndRelatedCostCostIncurredToDate1	8713000	usd
CY2023	us-gaap	Restructuring Charges RestructuringCharges	8713000	usd
CY2022Q4	us-gaap	Restructuring Reserve RestructuringReserve	0	usd
CY2022	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent Of Match DefinedContributionPlanEmployerMatchingContributionPercentOfMatch	1
CY2022	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent DefinedContributionPlanEmployerMatchingContributionPercent	0.04
CY2021	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent DefinedContributionPlanEmployerMatchingContributionPercent	0.04
CY2023	us-gaap	Defined Contribution Plan Cost Recognized DefinedContributionPlanCostRecognized	1300000	usd
CY2022	us-gaap	Defined Contribution Plan Cost Recognized DefinedContributionPlanCostRecognized	1100000	usd
CY2021	us-gaap	Defined Contribution Plan Cost Recognized DefinedContributionPlanCostRecognized	800000	usd
CY2023Q3	mrsn	Restructuring And Related Cost Number Of Positions To Be Eliminated Percent RestructuringAndRelatedCostNumberOfPositionsToBeEliminatedPercent	0.50
CY2023Q4	us-gaap	Restructuring And Related Cost Cost Incurred To Date1 RestructuringAndRelatedCostCostIncurredToDate1	8713000	usd
CY2023	us-gaap	Restructuring Charges RestructuringCharges	8713000	usd
CY2022Q4	us-gaap	Restructuring Reserve RestructuringReserve	0	usd
CY2023	mrsn	Restructuring Charges Additional Expense RestructuringChargesAdditionalExpense	7296000	usd
CY2023	us-gaap	Payments For Restructuring PaymentsForRestructuring	5917000	usd
CY2023	us-gaap	Restructuring Reserve Translation And Other Adjustment RestructuringReserveTranslationAndOtherAdjustment	-332000	usd
CY2023Q4	us-gaap	Restructuring Reserve RestructuringReserve	1047000	usd
CY2023Q4	ecd	Non Rule10b51 Arr Adopted Flag NonRule10b51ArrAdoptedFlag	false
CY2023Q4	ecd	Rule10b51 Arr Trmntd Flag Rule10b51ArrTrmntdFlag	false
CY2023Q4	ecd	Non Rule10b51 Arr Trmntd Flag NonRule10b51ArrTrmntdFlag	false
CY2023	mrsn	Restructuring Charges Additional Expense RestructuringChargesAdditionalExpense	7296000	usd
CY2023	us-gaap	Payments For Restructuring PaymentsForRestructuring	5917000	usd
CY2023	us-gaap	Restructuring Reserve Translation And Other Adjustment RestructuringReserveTranslationAndOtherAdjustment	-332000	usd
CY2023Q4	us-gaap	Restructuring Reserve RestructuringReserve	1047000	usd
CY2023Q4	ecd	Non Rule10b51 Arr Adopted Flag NonRule10b51ArrAdoptedFlag	false
CY2023Q4	ecd	Rule10b51 Arr Trmntd Flag Rule10b51ArrTrmntdFlag	false
CY2023Q4	ecd	Non Rule10b51 Arr Trmntd Flag NonRule10b51ArrTrmntdFlag	false

Files In Submission

Name	View Source	Status
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0001628280-24-007407-index.html	Edgar Link	pending
0001628280-24-007407.txt	Edgar Link	pending
0001628280-24-007407-xbrl.zip	Edgar Link	pending
exhibit1024-mbalaofferlett.htm	Edgar Link	pending
exhibit1043-mersanaxamende.htm	Edgar Link	pending
exhibit231-eyconsent.htm	Edgar Link	pending
exhibit31110kfy2023q4.htm	Edgar Link	pending
exhibit31210kfy2023q4.htm	Edgar Link	pending
exhibit32110kfy2023q4.htm	Edgar Link	pending
exhibit97-mersanaxamendeda.htm	Edgar Link	pending
Financial_Report.xlsx	Edgar Link	pending
logo1.jpg	Edgar Link	pending
MetaLinks.json	Edgar Link	pending
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mrsn-20231231.xsd	Edgar Link	pending
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mrsn-20231231_g5.jpg	Edgar Link	pending
mrsn-20231231_g6.jpg	Edgar Link	pending
mrsn-20231231_g7.jpg	Edgar Link	pending
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R22.htm	Edgar Link	pending
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R24.htm	Edgar Link	pending
R25.htm	Edgar Link	pending
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R9.htm	Edgar Link	pending
report.css	Edgar Link	pending
Show.js	Edgar Link	pending
FilingSummary.xml	Edgar Link	unprocessable
mrsn-20231231_cal.xml	Edgar Link	unprocessable
mrsn-20231231_pre.xml	Edgar Link	unprocessable
mrsn-20231231_lab.xml	Edgar Link	unprocessable

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$MRSN
Mersana Therapeutics, Inc. Stock $mrsn

2024 Q1 Form 10-K Financial Statement

#000162828024007407 Filed on February 28, 2024

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$MRSN Mersana Therapeutics, Inc. Stock $mrsn

2024 Q1 Form 10-K Financial Statement

#000162828024007407 Filed on February 28, 2024

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$MRSN
Mersana Therapeutics, Inc. Stock $mrsn