$MRSN
Mersana Therapeutics, Inc. Stock $mrsn

$mrsn Overview Financials Valuation SEC Filings Company Website

2024 Q4 Form 10-Q Financial Statement

#000162828024047441 Filed on November 13, 2024

Income Statement

Concept	2024 Q4	2024 Q3	2023 Q3
Revenue		$12.60M	$7.698M
YoY Change		63.65%	38.13%
Cost Of Revenue
YoY Change
Gross Profit
YoY Change
Gross Profit Margin
Selling, General & Admin		$9.864M	$12.89M
YoY Change		-23.48%	-11.55%
% of Gross Profit
Research & Development		$14.80M	$30.53M
YoY Change		-51.51%	-39.71%
% of Gross Profit
Depreciation & Amortization		$402.0K	$430.0K
YoY Change		-6.51%	101.88%
% of Gross Profit
Operating Expenses		$24.67M	$51.64M
YoY Change		-52.23%	-20.81%
Operating Profit
YoY Change
Interest Expense		$986.0K	$1.017M
YoY Change		-3.05%	15.57%
% of Operating Profit
Other Income/Expense, Net		$986.0K	$2.285M
YoY Change		-56.85%	-1428.49%
Pretax Income		-$11.08M	-$41.66M
YoY Change		-73.4%	-30.35%
Income Tax		$400.0K	$0.00
% Of Pretax Income
Net Earnings		-$11.50M	-$41.70M
YoY Change		-72.42%	-30.27%
Net Earnings / Revenue		-91.28%	-541.7%
Basic Earnings Per Share		-$0.09	-$0.35
Diluted Earnings Per Share		-$0.09	-$0.35
COMMON SHARES
Basic Shares Outstanding	123.5M shares	122.7M shares	120.5M shares
Diluted Shares Outstanding		122.7M shares	120.5M shares

Balance Sheet

Concept	2024 Q3	2023 Q3
SHORT-TERM ASSETS
Cash & Short-Term Investments	$155.2M	$241.0M
YoY Change	-35.61%	-16.93%
Cash & Equivalents	$78.87M	$186.3M
Short-Term Investments	$76.30M	$54.70M
Other Short-Term Assets	$4.386M	$8.800M
YoY Change	-50.16%	-11.21%
Inventory
Prepaid Expenses
Receivables	$1.000M	$0.00
Other Receivables	$0.00	$0.00
Total Short-Term Assets	$160.6M	$249.7M
YoY Change	-35.71%	-16.76%
LONG-TERM ASSETS
Property, Plant & Equipment	$7.995M	$12.70M
YoY Change	-37.05%	-10.54%
Goodwill
YoY Change
Intangibles
YoY Change
Long-Term Investments
YoY Change
Other Assets	$978.0K	$478.0K
YoY Change	104.6%	-22.4%
Total Long-Term Assets	$8.973M	$13.17M
YoY Change	-31.85%	-11.12%
TOTAL ASSETS
Total Short-Term Assets	$160.6M	$249.7M
Total Long-Term Assets	$8.973M	$13.17M
Total Assets	$169.5M	$262.9M
YoY Change	-35.52%	-16.5%
SHORT-TERM LIABILITIES
YoY Change
Accounts Payable	$2.033M	$12.95M
YoY Change	-84.3%	-5.72%
Accrued Expenses	$18.63M	$29.32M
YoY Change	-36.47%	-28.84%
Deferred Revenue	$31.37M	$21.59M
YoY Change	45.29%	-30.18%
Short-Term Debt	$0.00	$0.00
YoY Change
Long-Term Debt Due	$11.46M	$0.00
YoY Change
Total Short-Term Liabilities	$68.30M	$67.93M
YoY Change	0.54%	-23.49%
LONG-TERM LIABILITIES
Long-Term Debt	$14.00M	$25.00M
YoY Change	-44.0%	0.0%
Other Long-Term Liabilities	$21.00K	$67.00K
YoY Change	-68.66%	-74.81%
Total Long-Term Liabilities	$14.02M	$25.07M
YoY Change	-44.07%	-0.79%
TOTAL LIABILITIES
Total Short-Term Liabilities	$68.30M	$67.93M
Total Long-Term Liabilities	$14.02M	$25.07M
Total Liabilities	$168.5M	$210.7M
YoY Change	-20.04%	-6.19%
SHAREHOLDERS EQUITY
Retained Earnings	-$881.4M	-$806.8M
YoY Change	9.25%	32.31%
Common Stock	$882.5M	$859.0M
YoY Change	2.73%	22.68%
Preferred Stock
YoY Change
Treasury Stock (at cost)
YoY Change
Treasury Stock Shares
Shareholders Equity	$1.049M	$52.20M
YoY Change
Total Liabilities & Shareholders Equity	$169.5M	$262.9M
YoY Change	-35.52%	-16.5%

Cashflow Statement

Concept	2024 Q3	2023 Q3
OPERATING ACTIVITIES
Net Income	-$11.50M	-$41.70M
YoY Change	-72.42%	-30.27%
Depreciation, Depletion And Amortization	$402.0K	$430.0K
YoY Change	-6.51%	101.88%
Cash From Operating Activities	-$8.577M	-$46.08M
YoY Change	-81.39%	-184.4%
INVESTING ACTIVITIES
Capital Expenditures	$0.00	$470.0K
YoY Change	-100.0%	-210.33%
Acquisitions
YoY Change
Other Investing Activities	$28.47M	$104.3M
YoY Change	-72.69%	-748.93%
Cash From Investing Activities	$28.47M	$103.8M
YoY Change	-72.57%	-729.31%
FINANCING ACTIVITIES
Cash Dividend Paid
YoY Change
Common Stock Issuance & Retirement, Net
YoY Change
Debt Paid & Issued, Net
YoY Change
Cash From Financing Activities	-23.00K	-150.0K
YoY Change	-84.67%	-101.41%
NET CHANGE
Cash From Operating Activities	-8.577M	-46.08M
Cash From Investing Activities	28.47M	103.8M
Cash From Financing Activities	-23.00K	-150.0K
Net Change In Cash	19.87M	57.55M
YoY Change	-65.47%	18.07%
FREE CASH FLOW
Cash From Operating Activities	-$8.577M	-$46.08M
Capital Expenditures	$0.00	$470.0K
Free Cash Flow	-$8.577M	-$46.55M
YoY Change	-81.57%	-184.6%

Facts In Submission

Frame	Concept Type	Concept / XBRL Key	Value	Unit
	dei	Document Type DocumentType	10-Q
	dei	Document Quarterly Report DocumentQuarterlyReport	true
	dei	Document Period End Date DocumentPeriodEndDate	2024-09-30
	dei	Document Transition Report DocumentTransitionReport	false
	dei	Entity File Number EntityFileNumber	001-38129
	dei	Entity Registrant Name EntityRegistrantName	Mersana Therapeutics, Inc.
	dei	Entity Incorporation State Country Code EntityIncorporationStateCountryCode	DE
	dei	Entity Tax Identification Number EntityTaxIdentificationNumber	04-3562403
	dei	Entity Address Address Line1 EntityAddressAddressLine1	840 Memorial Drive
	dei	Entity Address City Or Town EntityAddressCityOrTown	Cambridge
	dei	Entity Address State Or Province EntityAddressStateOrProvince	MA
	dei	Entity Address Postal Zip Code EntityAddressPostalZipCode	02139
	dei	City Area Code CityAreaCode	617
	dei	Local Phone Number LocalPhoneNumber	498-0020
	dei	Security12b Title Security12bTitle	Common Stock, $0.0001 par value
	dei	Trading Symbol TradingSymbol	MRSN
	dei	Security Exchange Name SecurityExchangeName	NASDAQ
	dei	Entity Current Reporting Status EntityCurrentReportingStatus	Yes
	dei	Entity Interactive Data Current EntityInteractiveDataCurrent	Yes
	dei	Entity Filer Category EntityFilerCategory	Non-accelerated Filer
	dei	Entity Small Business EntitySmallBusiness	true
	dei	Entity Emerging Growth Company EntityEmergingGrowthCompany	false
	dei	Entity Shell Company EntityShellCompany	false
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	dei	Document Fiscal Year Focus DocumentFiscalYearFocus	2024
	dei	Document Fiscal Period Focus DocumentFiscalPeriodFocus	Q3
	dei	Amendment Flag AmendmentFlag	false
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CY2023Q3	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	-41656000	usd
	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	-54657000	usd
	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	-152126000	usd
CY2024Q3	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	418000	usd
CY2023Q3	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	0	usd
	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	418000	usd
	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	0	usd
CY2024Q3	us-gaap	Net Income Loss NetIncomeLoss	-11501000	usd
CY2023Q3	us-gaap	Net Income Loss NetIncomeLoss	-41656000	usd
	us-gaap	Net Income Loss NetIncomeLoss	-55075000	usd
	us-gaap	Net Income Loss NetIncomeLoss	-152126000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Availableforsale Securities Adjustment Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent	96000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Availableforsale Securities Adjustment Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent	50000	usd
	us-gaap	Other Comprehensive Income Availableforsale Securities Adjustment Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent	14000	usd
	us-gaap	Other Comprehensive Income Availableforsale Securities Adjustment Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent	153000	usd
CY2024Q3	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	-11405000	usd
CY2023Q3	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	-41606000	usd
	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	-55061000	usd
	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	-151973000	usd
CY2024Q3	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	-11501000	usd
CY2024Q3	us-gaap	Net Income Loss Available To Common Stockholders Diluted NetIncomeLossAvailableToCommonStockholdersDiluted	-11501000	usd
CY2023Q3	us-gaap	Net Income Loss Available To Common Stockholders Diluted NetIncomeLossAvailableToCommonStockholdersDiluted	-41656000	usd
CY2023Q3	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	-41656000	usd
	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	-55075000	usd
	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	-152126000	usd
	us-gaap	Net Income Loss Available To Common Stockholders Diluted NetIncomeLossAvailableToCommonStockholdersDiluted	-152126000	usd
CY2024Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-0.09
CY2024Q3	us-gaap	Liabilities Current LiabilitiesCurrent	68301000	usd
CY2023Q4	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	25000000	shares
CY2024Q3	us-gaap	Preferred Stock Shares Outstanding PreferredStockSharesOutstanding	0	shares
CY2024Q3	us-gaap	Preferred Stock Value PreferredStockValue	0	usd
	us-gaap	Restructuring Charges RestructuringCharges	0	usd
	us-gaap	Restructuring Charges RestructuringCharges	8214000	usd
	us-gaap	Operating Expenses OperatingExpenses	82661000	usd
	us-gaap	Operating Expenses OperatingExpenses	184397000	usd
CY2023Q3	us-gaap	Investment Income Interest InvestmentIncomeInterest	3302000	usd
	us-gaap	Investment Income Interest InvestmentIncomeInterest	6839000	usd
	us-gaap	Investment Income Interest InvestmentIncomeInterest	9142000	usd
CY2024Q3	us-gaap	Interest Expense Nonoperating InterestExpenseNonoperating	986000	usd
CY2023Q3	us-gaap	Interest Expense Nonoperating InterestExpenseNonoperating	1017000	usd
	us-gaap	Interest Expense Nonoperating InterestExpenseNonoperating	2971000	usd
	us-gaap	Interest Expense Nonoperating InterestExpenseNonoperating	3025000	usd
CY2023Q3	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	2285000	usd
	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	3868000	usd
CY2024Q3	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	-11083000	usd
CY2023Q3	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	-41656000	usd
	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	-54657000	usd
	us-gaap	Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	-152126000	usd
CY2024Q3	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	418000	usd
CY2023Q3	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	0	usd
	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	418000	usd
CY2023Q3	us-gaap	Net Income Loss NetIncomeLoss	-41656000	usd
	us-gaap	Net Income Loss NetIncomeLoss	-55075000	usd
	us-gaap	Net Income Loss NetIncomeLoss	-152126000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Availableforsale Securities Adjustment Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent	96000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Availableforsale Securities Adjustment Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent	50000	usd
	us-gaap	Other Comprehensive Income Availableforsale Securities Adjustment Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent	153000	usd
CY2024Q3	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	-11405000	usd
	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	-55061000	usd
	us-gaap	Comprehensive Income Net Of Tax ComprehensiveIncomeNetOfTax	-151973000	usd
CY2024Q3	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	-11501000	usd
CY2024Q3	us-gaap	Net Income Loss Available To Common Stockholders Diluted NetIncomeLossAvailableToCommonStockholdersDiluted	-11501000	usd
CY2023Q3	us-gaap	Net Income Loss Available To Common Stockholders Diluted NetIncomeLossAvailableToCommonStockholdersDiluted	-41656000	usd
CY2023Q3	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	-41656000	usd
	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	-55075000	usd
	us-gaap	Net Income Loss Available To Common Stockholders Diluted NetIncomeLossAvailableToCommonStockholdersDiluted	-55075000	usd
	us-gaap	Net Income Loss Available To Common Stockholders Basic NetIncomeLossAvailableToCommonStockholdersBasic	-152126000	usd
CY2023Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-0.35
CY2023Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-0.35
	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-0.45
	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-0.45
	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-1.33
	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-1.33
CY2023Q3	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	120521985	shares
CY2023Q3	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	120521985	shares
	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	122197585	shares
	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	122197585	shares
	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	114595910	shares
CY2023Q1	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	34000	usd
CY2023Q1	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	6407000	usd
CY2024Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-0.09
CY2023Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-0.35
CY2023Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-0.35
	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-0.45
	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-0.45
	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-1.33
	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-1.33
CY2024Q3	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	122721918	shares
CY2024Q3	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	122721918	shares
CY2023Q3	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	120521985	shares
CY2023Q3	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	120521985	shares
	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	122197585	shares
	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	122197585	shares
	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	114595910	shares
	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	114595910	shares
CY2022Q4	us-gaap	Stockholders Equity StockholdersEquity	92057000	usd
CY2023Q1	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	34000	usd
CY2023Q1	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	6407000	usd
CY2023Q1	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	164000	usd
CY2023Q1	us-gaap	Net Income Loss NetIncomeLoss	-56163000	usd
CY2023Q1	us-gaap	Stockholders Equity StockholdersEquity	64294000	usd
CY2023Q2	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	393000	usd
CY2023Q2	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Employee Stock Purchase Program Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationEmployeeStockPurchaseProgramRequisiteServicePeriodRecognition	963000	usd
CY2023Q2	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	6643000	usd
CY2023Q2	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-61000	usd
CY2023Q2	us-gaap	Net Income Loss NetIncomeLoss	-54307000	usd
CY2023Q2	us-gaap	Stockholders Equity StockholdersEquity	89800000	usd
CY2023Q3	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	4001000	usd
CY2023Q3	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	50000	usd
CY2023Q3	us-gaap	Net Income Loss NetIncomeLoss	-41656000	usd
CY2023Q4	us-gaap	Stockholders Equity StockholdersEquity	36904000	usd
CY2024Q1	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	48000	usd
CY2024Q1	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	4660000	usd
CY2024Q1	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-101000	usd
CY2024Q1	us-gaap	Net Income Loss NetIncomeLoss	-19306000	usd
CY2024Q1	us-gaap	Stockholders Equity StockholdersEquity	27985000	usd
CY2024Q2	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	8000	usd
CY2024Q2	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Employee Stock Purchase Program Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationEmployeeStockPurchaseProgramRequisiteServicePeriodRecognition	258000	usd
CY2024Q2	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	4425000	usd
CY2024Q2	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	19000	usd
CY2024Q2	us-gaap	Net Income Loss NetIncomeLoss	-24268000	usd
CY2024Q2	us-gaap	Stockholders Equity StockholdersEquity	8427000	usd
CY2024Q3	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	4027000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	96000	usd
CY2024Q3	us-gaap	Net Income Loss NetIncomeLoss	-11501000	usd
CY2024Q3	us-gaap	Stockholders Equity StockholdersEquity	1049000	usd
	us-gaap	Profit Loss ProfitLoss	-152126000	usd
	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	1070000	usd
	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	3335000	usd
	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	4239000	usd
	us-gaap	Share Based Compensation ShareBasedCompensation	17051000	usd
	mrsn	Noncash Operating Lease Expense NoncashOperatingLeaseExpense	2302000	usd
	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-230000	usd
CY2023Q1	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	164000	usd
CY2023Q2	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	393000	usd
CY2023Q2	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Employee Stock Purchase Program Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationEmployeeStockPurchaseProgramRequisiteServicePeriodRecognition	963000	usd
CY2023Q2	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	6643000	usd
CY2023Q2	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	-61000	usd
CY2023Q2	us-gaap	Net Income Loss NetIncomeLoss	-54307000	usd
CY2023Q2	us-gaap	Stockholders Equity StockholdersEquity	89800000	usd
CY2023Q3	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	4001000	usd
CY2023Q3	us-gaap	Stockholders Equity StockholdersEquity	52195000	usd
CY2023Q4	us-gaap	Stockholders Equity StockholdersEquity	36904000	usd
CY2024Q1	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	48000	usd
CY2024Q1	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	4660000	usd
CY2024Q1	us-gaap	Net Income Loss NetIncomeLoss	-19306000	usd
CY2024Q1	us-gaap	Stockholders Equity StockholdersEquity	27985000	usd
CY2024Q2	us-gaap	Stock Issued During Period Value Stock Options Exercised StockIssuedDuringPeriodValueStockOptionsExercised	8000	usd
CY2024Q2	us-gaap	Adjustments To Additional Paid In Capital Share Based Compensation Employee Stock Purchase Program Requisite Service Period Recognition AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationEmployeeStockPurchaseProgramRequisiteServicePeriodRecognition	258000	usd
CY2024Q2	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	4425000	usd
CY2024Q2	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	19000	usd
CY2024Q2	us-gaap	Net Income Loss NetIncomeLoss	-24268000	usd
CY2024Q2	us-gaap	Stockholders Equity StockholdersEquity	8427000	usd
CY2024Q3	us-gaap	Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue	4027000	usd
CY2024Q3	us-gaap	Other Comprehensive Income Loss Net Of Tax Portion Attributable To Parent OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	96000	usd
CY2024Q3	us-gaap	Net Income Loss NetIncomeLoss	-11501000	usd
CY2024Q3	us-gaap	Stockholders Equity StockholdersEquity	1049000	usd
	us-gaap	Profit Loss ProfitLoss	-152126000	usd
	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	1228000	usd
	us-gaap	Depreciation Depletion And Amortization DepreciationDepletionAndAmortization	1070000	usd
	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	4239000	usd
	us-gaap	Share Based Compensation ShareBasedCompensation	13112000	usd
	us-gaap	Share Based Compensation ShareBasedCompensation	17051000	usd
	mrsn	Noncash Operating Lease Expense NoncashOperatingLeaseExpense	2302000	usd
	mrsn	Noncash Operating Lease Expense NoncashOperatingLeaseExpense	2051000	usd
	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-230000	usd
	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-536000	usd
	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	1000000	usd
	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	-30000000	usd
	us-gaap	Increase Decrease In Prepaid Deferred Expense And Other Assets IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets	-585000	usd
	us-gaap	Increase Decrease In Prepaid Deferred Expense And Other Assets IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets	244000	usd
	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	500000	usd
	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	0	usd
	us-gaap	Increase Decrease In Accounts Payable IncreaseDecreaseInAccountsPayable	-5154000	usd
	us-gaap	Increase Decrease In Accounts Payable IncreaseDecreaseInAccountsPayable	-659000	usd
	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	-3271000	usd
	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	-13617000	usd
	us-gaap	Increase Decrease In Operating Lease Liability IncreaseDecreaseInOperatingLeaseLiability	-2353000	usd
	us-gaap	Increase Decrease In Operating Lease Liability IncreaseDecreaseInOperatingLeaseLiability	-2186000	usd
	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	418000	usd
	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	0	usd
	us-gaap	Increase Decrease In Contract With Customer Liability IncreaseDecreaseInContractWithCustomerLiability	-10276000	usd
	us-gaap	Increase Decrease In Contract With Customer Liability IncreaseDecreaseInContractWithCustomerLiability	-14557000	usd
	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	-63089000	usd
	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	-136920000	usd
	us-gaap	Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities	95000000	usd
	us-gaap	Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities	232970000	usd
	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	133431000	usd
	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-536000	usd
	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	1000000	usd
	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	-30000000	usd
	us-gaap	Increase Decrease In Prepaid Deferred Expense And Other Assets IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets	-585000	usd
	us-gaap	Increase Decrease In Prepaid Deferred Expense And Other Assets IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets	244000	usd
	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	500000	usd
	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	0	usd
	us-gaap	Increase Decrease In Accounts Payable IncreaseDecreaseInAccountsPayable	-5154000	usd
	us-gaap	Increase Decrease In Accounts Payable IncreaseDecreaseInAccountsPayable	-659000	usd
	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	-3271000	usd
	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	-13617000	usd
	us-gaap	Increase Decrease In Operating Lease Liability IncreaseDecreaseInOperatingLeaseLiability	-2353000	usd
	us-gaap	Increase Decrease In Operating Lease Liability IncreaseDecreaseInOperatingLeaseLiability	-2186000	usd
	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	418000	usd
	us-gaap	Increase Decrease In Other Operating Liabilities IncreaseDecreaseInOtherOperatingLiabilities	0	usd
	us-gaap	Increase Decrease In Contract With Customer Liability IncreaseDecreaseInContractWithCustomerLiability	-10276000	usd
	us-gaap	Increase Decrease In Contract With Customer Liability IncreaseDecreaseInContractWithCustomerLiability	-14557000	usd
	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	-63089000	usd
	us-gaap	Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities	95000000	usd
	us-gaap	Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities	232970000	usd
	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	133431000	usd
	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	131453000	usd
	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	132000	usd
	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	1782000	usd
	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-38563000	usd
	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	99735000	usd
	us-gaap	Proceeds From Issuance Of Common Stock ProceedsFromIssuanceOfCommonStock	5780000	usd
	us-gaap	Proceeds From Issuance Of Common Stock ProceedsFromIssuanceOfCommonStock	93539000	usd
	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	56000	usd
	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	427000	usd
	us-gaap	Proceeds From Issuance Of Shares Under Incentive And Share Based Compensation Plans ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlans	258000	usd
	us-gaap	Proceeds From Issuance Of Shares Under Incentive And Share Based Compensation Plans ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlans	963000	usd
	us-gaap	Payments Of Debt Issuance Costs PaymentsOfDebtIssuanceCosts	0	usd
	us-gaap	Payments Of Debt Issuance Costs PaymentsOfDebtIssuanceCosts	150000	usd
	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	135000	usd
	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	5959000	usd
	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	94583000	usd
	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	-95693000	usd
	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	57398000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	129363000	usd
CY2023Q3	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	186761000	usd
	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	0	usd
	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	442000	usd
	us-gaap	Interest Paid Net InterestPaidNet	2630000	usd
	us-gaap	Interest Paid Net InterestPaidNet	2508000	usd
CY2024Q3	mrsn	Number Of Additional Preclinical Candidates NumberOfAdditionalPreclinicalCandidates	2	candidate
CY2023Q3	us-gaap	Net Income Loss NetIncomeLoss	-41700000	usd
	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	57398000	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
CY2024Q3	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	79346000	usd
CY2023Q3	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	186761000	usd
	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	0	usd
	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	442000	usd
	us-gaap	Interest Paid Net InterestPaidNet	2630000	usd
	us-gaap	Interest Paid Net InterestPaidNet	2508000	usd
CY2024Q3	mrsn	Number Of Additional Preclinical Candidates NumberOfAdditionalPreclinicalCandidates	2	candidate
CY2024Q3	us-gaap	Net Income Loss NetIncomeLoss	-11500000	usd
CY2023Q3	us-gaap	Net Income Loss NetIncomeLoss	-41700000	usd
	us-gaap	Net Income Loss NetIncomeLoss	-55100000	usd
	us-gaap	Net Income Loss NetIncomeLoss	-152100000	usd
CY2024Q3	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	-881400000	usd
	us-gaap	Use Of Estimates UseOfEstimates	<div style="margin-bottom:12pt"><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-bottom:12pt"><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 unaudited condensed 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>
	us-gaap	Concentration Risk Credit Risk ConcentrationRiskCreditRisk	<div style="margin-bottom:12pt"><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>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.
	us-gaap	Net Income Loss NetIncomeLoss	-55100000	usd
	us-gaap	Net Income Loss NetIncomeLoss	-152100000	usd
CY2024Q3	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	-881400000	usd
	us-gaap	Use Of Estimates UseOfEstimates	<div style="margin-bottom:12pt"><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-bottom:12pt"><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 unaudited condensed 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>
	us-gaap	Concentration Risk Credit Risk ConcentrationRiskCreditRisk	<div style="margin-bottom:12pt"><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>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.
CY2023Q4	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	125314000	usd
	mrsn	Contract With Customer Liability Increase ContractWithCustomerLiabilityIncrease	5860000	usd
	mrsn	Contract With Customer Liability Decrease ContractWithCustomerLiabilityDecrease	15937000	usd
CY2024Q3	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	115237000	usd
CY2022Q4	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	147653000	usd
	mrsn	Contract With Customer Liability Increase ContractWithCustomerLiabilityIncrease	1710000	usd
	mrsn	Contract With Customer Liability Decrease ContractWithCustomerLiabilityDecrease	16267000	usd
CY2023Q3	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	133096000	usd
CY2023Q3	us-gaap	Contract With Customer Liability Revenue Recognized ContractWithCustomerLiabilityRevenueRecognized	4503000	usd
	us-gaap	Contract With Customer Liability Revenue Recognized ContractWithCustomerLiabilityRevenueRecognized	14875000	usd
	us-gaap	Contract With Customer Liability Revenue Recognized ContractWithCustomerLiabilityRevenueRecognized	17963000	usd
CY2023Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	174561000	usd
CY2024Q3	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	78868000	usd
CY2022Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	128885000	usd
CY2023Q3	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	186283000	usd
CY2023Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2024Q3	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2022Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2023Q3	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2023Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	175039000	usd
CY2024Q3	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	79346000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	129363000	usd
CY2023Q3	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	186761000	usd
CY2023Q4	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	125314000	usd
	mrsn	Contract With Customer Liability Increase ContractWithCustomerLiabilityIncrease	5860000	usd
	mrsn	Contract With Customer Liability Decrease ContractWithCustomerLiabilityDecrease	15937000	usd
CY2024Q3	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	115237000	usd
CY2022Q4	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	147653000	usd
	mrsn	Contract With Customer Liability Increase ContractWithCustomerLiabilityIncrease	1710000	usd
	mrsn	Contract With Customer Liability Decrease ContractWithCustomerLiabilityDecrease	16267000	usd
CY2023Q3	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	133096000	usd
CY2024Q3	us-gaap	Contract With Customer Liability Revenue Recognized ContractWithCustomerLiabilityRevenueRecognized	4388000	usd
CY2023Q3	us-gaap	Contract With Customer Liability Revenue Recognized ContractWithCustomerLiabilityRevenueRecognized	4503000	usd
	us-gaap	Contract With Customer Liability Revenue Recognized ContractWithCustomerLiabilityRevenueRecognized	14875000	usd
	us-gaap	Contract With Customer Liability Revenue Recognized ContractWithCustomerLiabilityRevenueRecognized	17963000	usd
CY2023Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	174561000	usd
CY2024Q3	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	78868000	usd
CY2022Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	128885000	usd
CY2023Q3	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	186283000	usd
CY2023Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2024Q3	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2022Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2023Q3	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	478000	usd
CY2023Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	175039000	usd
CY2024Q3	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	79346000	usd
CY2022Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	129363000	usd
CY2023Q3	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	186761000	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
CY2024Q3	us-gaap	Debt Securities Available For Sale Unrealized Loss Position Number Of Positions DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions	2	security
CY2024Q3	us-gaap	Debt Securities Available For Sale Unrealized Loss Position DebtSecuritiesAvailableForSaleUnrealizedLossPosition	17700000	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position12 Months Or Longer Number Of Positions DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions	0	security
	us-gaap	Debt Securities Available For Sale Excluding Accrued Interest Allowance For Credit Loss Period Increase Decrease DebtSecuritiesAvailableForSaleExcludingAccruedInterestAllowanceForCreditLossPeriodIncreaseDecrease	0	usd
	us-gaap	Debt Securities Available For Sale Excluding Accrued Interest Allowance For Credit Loss Period Increase Decrease DebtSecuritiesAvailableForSaleExcludingAccruedInterestAllowanceForCreditLossPeriodIncreaseDecrease	0	usd
CY2024Q3	us-gaap	Employee Related Liabilities Current EmployeeRelatedLiabilitiesCurrent	10207000	usd
CY2023Q4	us-gaap	Employee Related Liabilities Current EmployeeRelatedLiabilitiesCurrent	8807000	usd
CY2024Q3	mrsn	Accrued Clinical Expenses Current AccruedClinicalExpensesCurrent	2843000	usd
CY2023Q4	mrsn	Accrued Clinical Expenses Current AccruedClinicalExpensesCurrent	5063000	usd
CY2024Q3	mrsn	Accrued Manufacturing Expenses Current AccruedManufacturingExpensesCurrent	2452000	usd
CY2023Q4	mrsn	Accrued Manufacturing Expenses Current AccruedManufacturingExpensesCurrent	2566000	usd
CY2024Q3	mrsn	Accrued Research Development And Non Clinical Expenses Current AccruedResearchDevelopmentAndNonClinicalExpensesCurrent	1879000	usd
CY2023Q4	mrsn	Accrued Research Development And Non Clinical Expenses Current AccruedResearchDevelopmentAndNonClinicalExpensesCurrent	3090000	usd
CY2024Q3	us-gaap	Accrued Professional Fees Current AccruedProfessionalFeesCurrent	930000	usd
CY2023Q4	us-gaap	Accrued Professional Fees Current AccruedProfessionalFeesCurrent	936000	usd
CY2024Q3	us-gaap	Other Accrued Liabilities Current OtherAccruedLiabilitiesCurrent	317000	usd
CY2023Q4	us-gaap	Other Accrued Liabilities Current OtherAccruedLiabilitiesCurrent	389000	usd
CY2024Q3	us-gaap	Restructuring Reserve Current RestructuringReserveCurrent	0	usd
CY2023Q4	us-gaap	Restructuring Reserve Current RestructuringReserveCurrent	1047000	usd
CY2024Q3	us-gaap	Accrued Liabilities Current AccruedLiabilitiesCurrent	18628000	usd
CY2023Q4	us-gaap	Accrued Liabilities Current AccruedLiabilitiesCurrent	21898000	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
CY2024Q3	us-gaap	Debt Securities Available For Sale Unrealized Loss Position Number Of Positions DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions	2	security
CY2024Q3	us-gaap	Debt Securities Available For Sale Unrealized Loss Position DebtSecuritiesAvailableForSaleUnrealizedLossPosition	17700000	usd
CY2024Q3	us-gaap	Debt Securities Available For Sale Continuous Unrealized Loss Position12 Months Or Longer Number Of Positions DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions	0	security
	us-gaap	Debt Securities Available For Sale Excluding Accrued Interest Allowance For Credit Loss Period Increase Decrease DebtSecuritiesAvailableForSaleExcludingAccruedInterestAllowanceForCreditLossPeriodIncreaseDecrease	0	usd
	us-gaap	Debt Securities Available For Sale Excluding Accrued Interest Allowance For Credit Loss Period Increase Decrease DebtSecuritiesAvailableForSaleExcludingAccruedInterestAllowanceForCreditLossPeriodIncreaseDecrease	0	usd
CY2023Q4	us-gaap	Employee Related Liabilities Current EmployeeRelatedLiabilitiesCurrent	8807000	usd
CY2024Q3	mrsn	Accrued Clinical Expenses Current AccruedClinicalExpensesCurrent	2843000	usd
CY2023Q4	mrsn	Accrued Clinical Expenses Current AccruedClinicalExpensesCurrent	5063000	usd
CY2024Q3	mrsn	Accrued Manufacturing Expenses Current AccruedManufacturingExpensesCurrent	2452000	usd
CY2023Q4	mrsn	Accrued Manufacturing Expenses Current AccruedManufacturingExpensesCurrent	2566000	usd
CY2024Q3	mrsn	Accrued Research Development And Non Clinical Expenses Current AccruedResearchDevelopmentAndNonClinicalExpensesCurrent	1879000	usd
CY2023Q4	mrsn	Accrued Research Development And Non Clinical Expenses Current AccruedResearchDevelopmentAndNonClinicalExpensesCurrent	3090000	usd
CY2024Q3	us-gaap	Accrued Professional Fees Current AccruedProfessionalFeesCurrent	930000	usd
CY2023Q4	us-gaap	Accrued Professional Fees Current AccruedProfessionalFeesCurrent	936000	usd
CY2024Q3	us-gaap	Other Accrued Liabilities Current OtherAccruedLiabilitiesCurrent	317000	usd
CY2024Q3	us-gaap	Restructuring Reserve Current RestructuringReserveCurrent	0	usd
CY2023Q4	us-gaap	Restructuring Reserve Current RestructuringReserveCurrent	1047000	usd
CY2024Q3	us-gaap	Accrued Liabilities Current AccruedLiabilitiesCurrent	18628000	usd
CY2023Q4	us-gaap	Accrued Liabilities Current AccruedLiabilitiesCurrent	21898000	usd
CY2024Q3	us-gaap	Debt Instrument Carrying Amount DebtInstrumentCarryingAmount	25000000	usd
CY2024Q3	us-gaap	Long Term Debt Current LongTermDebtCurrent	11458000	usd
CY2024Q3	mrsn	Long Term Debt Noncurrent Before Adjustments LongTermDebtNoncurrentBeforeAdjustments	13542000	usd
CY2024Q3	us-gaap	Debt Instrument Unamortized Discount DebtInstrumentUnamortizedDiscount	173000	usd
CY2024Q3	us-gaap	Debt Instrument Unamortized Premium DebtInstrumentUnamortizedPremium	630000	usd
CY2024Q3	us-gaap	Long Term Debt Noncurrent LongTermDebtNoncurrent	13999000	usd
CY2024Q3	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2024Q3	us-gaap	Common Stock Capital Shares Reserved For Future Issuance CommonStockCapitalSharesReservedForFutureIssuance	17274942	shares
CY2023Q4	us-gaap	Common Stock Capital Shares Reserved For Future Issuance CommonStockCapitalSharesReservedForFutureIssuance	14736953	shares
CY2024Q3	us-gaap	Common Stock Capital Shares Reserved For Future Issuance CommonStockCapitalSharesReservedForFutureIssuance	17274942	shares
CY2023Q4	us-gaap	Common Stock Capital Shares Reserved For Future Issuance CommonStockCapitalSharesReservedForFutureIssuance	14736953	shares
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
	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Gross ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross	2687497	shares
	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Grants In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice	2.91
	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	14223	shares
	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Exercises In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice	3.25
	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Expirations In Period ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod	1048874	shares
	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Expirations In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice	7.98
CY2024Q3	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	12527245	shares
CY2024Q3	us-gaap	Debt Instrument Carrying Amount DebtInstrumentCarryingAmount	25000000	usd
CY2024Q3	us-gaap	Long Term Debt Current LongTermDebtCurrent	11458000	usd
CY2024Q3	mrsn	Long Term Debt Noncurrent Before Adjustments LongTermDebtNoncurrentBeforeAdjustments	13542000	usd
CY2024Q3	us-gaap	Debt Instrument Unamortized Discount DebtInstrumentUnamortizedDiscount	173000	usd
CY2024Q3	us-gaap	Debt Instrument Unamortized Premium DebtInstrumentUnamortizedPremium	630000	usd
CY2024Q3	us-gaap	Long Term Debt Noncurrent LongTermDebtNoncurrent	13999000	usd
CY2024Q3	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	25000000	shares
CY2024Q3	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2024Q3	us-gaap	Common Stock Capital Shares Reserved For Future Issuance CommonStockCapitalSharesReservedForFutureIssuance	17274942	shares
CY2023Q4	us-gaap	Common Stock Capital Shares Reserved For Future Issuance CommonStockCapitalSharesReservedForFutureIssuance	14736953	shares
CY2024Q3	us-gaap	Common Stock Capital Shares Reserved For Future Issuance CommonStockCapitalSharesReservedForFutureIssuance	17274942	shares
CY2023Q4	us-gaap	Common Stock Capital Shares Reserved For Future Issuance CommonStockCapitalSharesReservedForFutureIssuance	14736953	shares
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
	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Gross ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross	2687497	shares
	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Grants In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice	2.91
	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	14223	shares
	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Exercises In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice	3.25
	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Expirations In Period ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod	1048874	shares
	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Expirations In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice	7.98
CY2024Q3	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	12527245	shares
CY2024Q3	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	6.41
CY2024Q3	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber	7716813	shares
CY2024Q3	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice	7.81
	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	2.50
	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	3.85
	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	300000	usd
	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	400000	usd
CY2024Q3	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	6.41
CY2024Q3	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber	7716813	shares
CY2024Q3	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice	7.81
	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	2.50
	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	3.85
	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	300000	usd
	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	400000	usd
CY2024Q3	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	17274942	shares
	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	17274942	shares
	us-gaap	Research And Development Arrangement Contract To Perform For Others Costs Incurred Gross ResearchAndDevelopmentArrangementContractToPerformForOthersCostsIncurredGross	0	usd
CY2023Q3	us-gaap	Research And Development Arrangement Contract To Perform For Others Costs Incurred Gross ResearchAndDevelopmentArrangementContractToPerformForOthersCostsIncurredGross	0	usd
	us-gaap	Research And Development Arrangement Contract To Perform For Others Costs Incurred Gross ResearchAndDevelopmentArrangementContractToPerformForOthersCostsIncurredGross	0	usd
CY2023Q3	mrsn	Restructuring And Related Cost Number Of Positions To Be Eliminated Percent RestructuringAndRelatedCostNumberOfPositionsToBeEliminatedPercent	0.50
CY2024Q3	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	17274942	shares
	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	17274942	shares
	us-gaap	Research And Development Arrangement Contract To Perform For Others Costs Incurred Gross ResearchAndDevelopmentArrangementContractToPerformForOthersCostsIncurredGross	0	usd
CY2024Q3	us-gaap	Research And Development Arrangement Contract To Perform For Others Costs Incurred Gross ResearchAndDevelopmentArrangementContractToPerformForOthersCostsIncurredGross	0	usd
CY2023Q3	mrsn	Restructuring And Related Cost Number Of Positions To Be Eliminated Percent RestructuringAndRelatedCostNumberOfPositionsToBeEliminatedPercent	0.50
CY2024Q3	us-gaap	Restructuring And Related Cost Cost Incurred To Date1 RestructuringAndRelatedCostCostIncurredToDate1	8713000	usd
	us-gaap	Restructuring Charges RestructuringCharges	8214000	usd
CY2023Q3	us-gaap	Restructuring Charges RestructuringCharges	8214000	usd
	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	400000	usd
CY2024Q3	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	400000	usd
CY2024Q3	ecd	Rule10b51 Arr Adopted Flag Rule10b51ArrAdoptedFlag	false
CY2024Q3	ecd	Non Rule10b51 Arr Adopted Flag NonRule10b51ArrAdoptedFlag	false
CY2024Q3	ecd	Rule10b51 Arr Trmntd Flag Rule10b51ArrTrmntdFlag	false
CY2024Q3	ecd	Non Rule10b51 Arr Trmntd Flag NonRule10b51ArrTrmntdFlag	false
CY2024Q3	us-gaap	Restructuring And Related Cost Cost Incurred To Date1 RestructuringAndRelatedCostCostIncurredToDate1	8713000	usd
	us-gaap	Restructuring Charges RestructuringCharges	8214000	usd
CY2023Q3	us-gaap	Restructuring Charges RestructuringCharges	8214000	usd
CY2024Q3	us-gaap	Income Tax Expense Benefit IncomeTaxExpenseBenefit	400000	usd
CY2024Q3	ecd	Rule10b51 Arr Adopted Flag Rule10b51ArrAdoptedFlag	false
CY2024Q3	ecd	Non Rule10b51 Arr Adopted Flag NonRule10b51ArrAdoptedFlag	false
CY2024Q3	ecd	Rule10b51 Arr Trmntd Flag Rule10b51ArrTrmntdFlag	false
CY2024Q3	ecd	Non Rule10b51 Arr Trmntd Flag NonRule10b51ArrTrmntdFlag	false

Files In Submission

Name	View Source	Status
0001628280-24-047441-index-headers.html	Edgar Link	pending
0001628280-24-047441-index.html	Edgar Link	pending
0001628280-24-047441.txt	Edgar Link	pending
0001628280-24-047441-xbrl.zip	Edgar Link	pending
exhibit31110-qfy2024q3.htm	Edgar Link	pending
exhibit31210-qfy2024q3.htm	Edgar Link	pending
exhibit32110-qfy2024q3.htm	Edgar Link	pending
Financial_Report.xlsx	Edgar Link	pending
MetaLinks.json	Edgar Link	pending
mrsn-20240930.htm	Edgar Link	pending
mrsn-20240930.xsd	Edgar Link	pending
R1.htm	Edgar Link	pending
R10.htm	Edgar Link	pending
R11.htm	Edgar Link	pending
R12.htm	Edgar Link	pending
R13.htm	Edgar Link	pending
R14.htm	Edgar Link	pending
R15.htm	Edgar Link	pending
R16.htm	Edgar Link	pending
R17.htm	Edgar Link	pending
R18.htm	Edgar Link	pending
R19.htm	Edgar Link	pending
R2.htm	Edgar Link	pending
R20.htm	Edgar Link	pending
R21.htm	Edgar Link	pending
R22.htm	Edgar Link	pending
R23.htm	Edgar Link	pending
R24.htm	Edgar Link	pending
R25.htm	Edgar Link	pending
R26.htm	Edgar Link	pending
R27.htm	Edgar Link	pending
R28.htm	Edgar Link	pending
R29.htm	Edgar Link	pending
R3.htm	Edgar Link	pending
R30.htm	Edgar Link	pending
R31.htm	Edgar Link	pending
R32.htm	Edgar Link	pending
R33.htm	Edgar Link	pending
R34.htm	Edgar Link	pending
R35.htm	Edgar Link	pending
R36.htm	Edgar Link	pending
R37.htm	Edgar Link	pending
R38.htm	Edgar Link	pending
R39.htm	Edgar Link	pending
R4.htm	Edgar Link	pending
R40.htm	Edgar Link	pending
R41.htm	Edgar Link	pending
mrsn-20240930_cal.xml	Edgar Link	unprocessable
mrsn-20240930_def.xml	Edgar Link	unprocessable
mrsn-20240930_lab.xml	Edgar Link	unprocessable
mrsn-20240930_pre.xml	Edgar Link	unprocessable
R42.htm	Edgar Link	pending
R43.htm	Edgar Link	pending
R44.htm	Edgar Link	pending
R45.htm	Edgar Link	pending
R46.htm	Edgar Link	pending
R47.htm	Edgar Link	pending
R48.htm	Edgar Link	pending
R49.htm	Edgar Link	pending
R5.htm	Edgar Link	pending
R50.htm	Edgar Link	pending
R51.htm	Edgar Link	pending
R52.htm	Edgar Link	pending
R53.htm	Edgar Link	pending
R54.htm	Edgar Link	pending
R55.htm	Edgar Link	pending
R56.htm	Edgar Link	pending
R57.htm	Edgar Link	pending
R58.htm	Edgar Link	pending
R59.htm	Edgar Link	pending
R6.htm	Edgar Link	pending
R60.htm	Edgar Link	pending
R7.htm	Edgar Link	pending
R8.htm	Edgar Link	pending
R9.htm	Edgar Link	pending
report.css	Edgar Link	pending
Show.js	Edgar Link	pending
FilingSummary.xml	Edgar Link	unprocessable
mrsn-20240930_htm.xml	Edgar Link	completed

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$MRSN
Mersana Therapeutics, Inc. Stock $mrsn

2024 Q4 Form 10-Q Financial Statement

#000162828024047441 Filed on November 13, 2024

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$MRSN Mersana Therapeutics, Inc. Stock $mrsn

2024 Q4 Form 10-Q Financial Statement

#000162828024047441 Filed on November 13, 2024

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$MRSN
Mersana Therapeutics, Inc. Stock $mrsn