$MRSN
Mersana Therapeutics, Inc. Stock $mrsn

$mrsn Overview Financials Valuation SEC Filings Company Website

2020 Q4 Form 10-K Financial Statement

#000162828021003370 Filed on February 26, 2021

Income Statement

Concept	2020 Q4	2019 Q4	2019 Q3
Revenue	$10.00K	$42.00K	$844.0K
YoY Change	-76.19%	-96.47%	-60.74%
Cost Of Revenue
YoY Change
Gross Profit
YoY Change
Gross Profit Margin
Selling, General & Admin	$5.910M	$4.210M	$4.440M
YoY Change	40.38%	1.45%	1.37%
% of Gross Profit
Research & Development	$22.86M	$12.43M	$13.70M
YoY Change	83.89%	-37.29%	-9.74%
% of Gross Profit
Depreciation & Amortization	$260.0K	$290.0K	$310.0K
YoY Change	-10.34%	-17.14%	-8.82%
% of Gross Profit
Operating Expenses	$28.77M	$16.64M	$18.14M
YoY Change	72.89%	-30.57%	-7.28%
Operating Profit
YoY Change
Interest Expense	$92.00K	$87.00K	$107.0K
YoY Change	5.75%	-75.14%	-68.53%
% of Operating Profit
Other Income/Expense, Net	-$82.00K	$354.0K	$501.0K
YoY Change	-123.16%
Pretax Income	-$28.84M	-$16.25M	-$16.79M
YoY Change	77.48%	-27.55%	-1.64%
Income Tax
% Of Pretax Income
Net Earnings	-$28.84M	-$16.25M	-$16.79M
YoY Change	77.55%	-27.57%	-1.63%
Net Earnings / Revenue	-288440.0%	-38680.95%	-1989.57%
Basic Earnings Per Share	-$0.42	-$0.34	-$0.35
Diluted Earnings Per Share	-$0.42	-$0.34	-$0.35
COMMON SHARES
Basic Shares Outstanding	68.50M shares	47.89M shares	47.83M shares
Diluted Shares Outstanding		47.89M shares	47.83M shares

Balance Sheet

Concept	2020 Q4	2019 Q4	2019 Q3
SHORT-TERM ASSETS
Cash & Short-Term Investments	$255.1M	$99.79M	$112.0M
YoY Change	155.63%	42.35%	30.08%
Cash & Equivalents	$255.1M	$62.35M	$58.20M
Short-Term Investments	$0.00	$37.44M	$53.80M
Other Short-Term Assets	$3.500M	$1.500M	$2.400M
YoY Change	133.33%	-59.46%	-29.41%
Inventory
Prepaid Expenses
Receivables		$0.00	$300.0K
Other Receivables		$0.00	$0.00
Total Short-Term Assets	$258.6M	$101.3M	$114.7M
YoY Change	155.2%	36.37%	27.44%
LONG-TERM ASSETS
Property, Plant & Equipment	$1.730M	$2.164M	$5.500M
YoY Change	-20.06%	-19.85%	120.0%
Goodwill
YoY Change
Intangibles
YoY Change
Long-Term Investments
YoY Change
Other Assets	$2.153M	$1.453M	$1.500M
YoY Change	48.18%	-3.13%	-16.67%
Total Long-Term Assets	$14.82M	$6.215M	$7.000M
YoY Change	138.44%	47.98%	59.09%
TOTAL ASSETS
Total Short-Term Assets	$258.6M	$101.3M	$114.7M
Total Long-Term Assets	$14.82M	$6.215M	$7.000M
Total Assets	$273.4M	$107.5M	$121.7M
YoY Change	154.23%	36.99%	28.92%
SHORT-TERM LIABILITIES
YoY Change
Accounts Payable	$8.340M	$7.296M	$7.300M
YoY Change	14.31%	-31.81%	8.96%
Accrued Expenses	$16.15M	$8.986M	$10.20M
YoY Change	79.68%	-27.53%	-2.86%
Deferred Revenue	$3.987M	$4.815M
YoY Change	-17.2%
Short-Term Debt	$0.00	$0.00	$0.00
YoY Change
Long-Term Debt Due	$0.00	$667.0K	$300.0K
YoY Change	-100.0%
Total Short-Term Liabilities	$30.00M	$24.07M	$22.40M
YoY Change	24.65%	-65.32%	-28.66%
LONG-TERM LIABILITIES
Long-Term Debt	$4.977M	$4.201M	$5.000M
YoY Change	18.47%
Other Long-Term Liabilities	$174.0K	$275.0K	$1.300M
YoY Change	-36.73%	-8.33%	-96.07%
Total Long-Term Liabilities	$5.151M	$4.476M	$6.300M
YoY Change	15.08%	1392.0%	-80.97%
TOTAL LIABILITIES
Total Short-Term Liabilities	$30.00M	$24.07M	$22.40M
Total Long-Term Liabilities	$5.151M	$4.476M	$6.300M
Total Liabilities	$45.31M	$29.22M	$28.60M
YoY Change	55.06%	-58.07%	-55.66%
SHAREHOLDERS EQUITY
Retained Earnings	-$280.4M	-$192.4M
YoY Change	45.77%
Common Stock	$508.5M	$270.7M
YoY Change	87.87%
Preferred Stock
YoY Change
Treasury Stock (at cost)
YoY Change
Treasury Stock Shares
Shareholders Equity	$228.1M	$78.32M	$93.05M
YoY Change
Total Liabilities & Shareholders Equity	$273.4M	$107.5M	$121.7M
YoY Change	154.23%	36.99%	28.92%

Cashflow Statement

Concept	2020 Q4	2019 Q4	2019 Q3
OPERATING ACTIVITIES
Net Income	-$28.84M	-$16.25M	-$16.79M
YoY Change	77.55%	-27.57%	-1.63%
Depreciation, Depletion And Amortization	$260.0K	$290.0K	$310.0K
YoY Change	-10.34%	-17.14%	-8.82%
Cash From Operating Activities	-$17.32M	-$12.57M	-$16.29M
YoY Change	37.79%	-20.99%	55.59%
INVESTING ACTIVITIES
Capital Expenditures	-$180.0K	$0.00	-$30.00K
YoY Change		-100.0%	-88.89%
Acquisitions
YoY Change
Other Investing Activities	$0.00	$16.50M	-$41.74M
YoY Change	-100.0%	17.86%	-298.76%
Cash From Investing Activities	-$190.0K	$16.50M	-$41.76M
YoY Change	-101.15%	20.17%	-301.54%
FINANCING ACTIVITIES
Cash Dividend Paid
YoY Change
Common Stock Issuance & Retirement, Net
YoY Change
Debt Paid & Issued, Net
YoY Change
Cash From Financing Activities	1.660M	230.0K	-10.00K
YoY Change	621.74%	9.52%	-104.76%
NET CHANGE
Cash From Operating Activities	-17.32M	-12.57M	-16.29M
Cash From Investing Activities	-190.0K	16.50M	-41.76M
Cash From Financing Activities	1.660M	230.0K	-10.00K
Net Change In Cash	-15.85M	4.160M	-58.06M
YoY Change	-481.01%	-311.17%	-655.07%
FREE CASH FLOW
Cash From Operating Activities	-$17.32M	-$12.57M	-$16.29M
Capital Expenditures	-$180.0K	$0.00	-$30.00K
Free Cash Flow	-$17.14M	-$12.57M	-$16.26M
YoY Change	36.36%	-19.58%	59.41%

Facts In Submission

Frame	Concept Type	Concept / XBRL Key	Value	Unit
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CY2020	dei	Document Fiscal Year Focus DocumentFiscalYearFocus	2020
CY2020	dei	Document Fiscal Period Focus DocumentFiscalPeriodFocus	FY
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CY2020	dei	Document Annual Report DocumentAnnualReport	true
CY2020	dei	Document Period End Date DocumentPeriodEndDate	2020-12-31
CY2020	dei	Current Fiscal Year End Date CurrentFiscalYearEndDate	--12-31
CY2020	dei	Document Transition Report DocumentTransitionReport	false
CY2020	dei	Entity File Number EntityFileNumber	001-38129
CY2020	dei	Entity Registrant Name EntityRegistrantName	Mersana Therapeutics, Inc.
CY2020	dei	Entity Incorporation State Country Code EntityIncorporationStateCountryCode	DE
CY2020	dei	Entity Tax Identification Number EntityTaxIdentificationNumber	04-3562403
CY2020	dei	Entity Address Address Line1 EntityAddressAddressLine1	840 Memorial Drive
CY2020	dei	Entity Address City Or Town EntityAddressCityOrTown	Cambridge
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CY2019	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	0	USD
CY2018	us-gaap	Gain Loss On Sale Of Property Plant Equipment GainLossOnSaleOfPropertyPlantEquipment	-20000	USD
CY2020	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	86000	USD
CY2019	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	222000	USD
CY2018	us-gaap	Accretion Amortization Of Discounts And Premiums Investments AccretionAmortizationOfDiscountsAndPremiumsInvestments	296000	USD
CY2020	us-gaap	Share Based Compensation ShareBasedCompensation	7172000	USD
CY2019	us-gaap	Share Based Compensation ShareBasedCompensation	4872000	USD
CY2018	us-gaap	Share Based Compensation ShareBasedCompensation	3884000	USD
CY2020	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	0	USD
CY2019	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	-459000	USD
CY2018	us-gaap	Increase Decrease In Accounts Receivable IncreaseDecreaseInAccountsReceivable	-325000	USD
CY2020	us-gaap	Increase Decrease In Prepaid Deferred Expense And Other Assets IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets	1950000	USD
CY2020	mrsn	Change In Deferred Rent ChangeInDeferredRent	0	USD
CY2019	mrsn	Change In Deferred Rent ChangeInDeferredRent	0	USD
CY2018	mrsn	Change In Deferred Rent ChangeInDeferredRent	110000	USD
CY2020	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-148000	USD
CY2019	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	-103000	USD
CY2018	us-gaap	Other Noncash Income Expense OtherNoncashIncomeExpense	0	USD
CY2019	us-gaap	Increase Decrease In Prepaid Deferred Expense And Other Assets IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets	-2179000	USD
CY2018	us-gaap	Increase Decrease In Prepaid Deferred Expense And Other Assets IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets	1690000	USD
CY2020	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	700000	USD
CY2019	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	0	USD
CY2018	us-gaap	Increase Decrease In Other Operating Assets IncreaseDecreaseInOtherOperatingAssets	1132000	USD
CY2020	us-gaap	Increase Decrease In Accounts Payable IncreaseDecreaseInAccountsPayable	942000	USD
CY2019	us-gaap	Increase Decrease In Accounts Payable IncreaseDecreaseInAccountsPayable	-3110000	USD
CY2018	us-gaap	Increase Decrease In Accounts Payable IncreaseDecreaseInAccountsPayable	7375000	USD
CY2020	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	7280000	USD
CY2019	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	-3569000	USD
CY2018	us-gaap	Increase Decrease In Accrued Liabilities IncreaseDecreaseInAccruedLiabilities	5431000	USD
CY2020	mrsn	Increase Decrease In Operating Lease Assets IncreaseDecreaseInOperatingLeaseAssets	-1642000	USD
CY2019	mrsn	Increase Decrease In Operating Lease Assets IncreaseDecreaseInOperatingLeaseAssets	-1771000	USD
CY2018	mrsn	Increase Decrease In Operating Lease Assets IncreaseDecreaseInOperatingLeaseAssets	0	USD
CY2020	mrsn	Increase Decrease In Operating Lease Liabilities IncreaseDecreaseInOperatingLeaseLiabilities	-1281000	USD
CY2019	mrsn	Increase Decrease In Operating Lease Liabilities IncreaseDecreaseInOperatingLeaseLiabilities	-1883000	USD
CY2018	mrsn	Increase Decrease In Operating Lease Liabilities IncreaseDecreaseInOperatingLeaseLiabilities	0	USD
CY2020	us-gaap	Increase Decrease In Contract With Customer Liability IncreaseDecreaseInContractWithCustomerLiability	-828000	USD
CY2019	us-gaap	Increase Decrease In Contract With Customer Liability IncreaseDecreaseInContractWithCustomerLiability	-41381000	USD
CY2018	us-gaap	Increase Decrease In Contract With Customer Liability IncreaseDecreaseInContractWithCustomerLiability	-6243000	USD
CY2020	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	-74696000	USD
CY2019	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	-67744000	USD
CY2018	us-gaap	Net Cash Provided By Used In Operating Activities NetCashProvidedByUsedInOperatingActivities	-55216000	USD
CY2020	us-gaap	Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities	37500000	USD
CY2019	us-gaap	Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities	27000000	USD
CY2018	us-gaap	Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities	88565000	USD
CY2020	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	0	USD
CY2019	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	53688000	USD
CY2018	us-gaap	Proceeds From Issuance Of Shares Under Incentive And Share Based Compensation Plans ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlans	146000	USD
CY2020	mrsn	Proceeds From Debt Net Issuance Costs ProceedsFromDebtNetIssuanceCosts	-197000	USD
CY2018	us-gaap	Payments To Acquire Available For Sale Securities Debt PaymentsToAcquireAvailableForSaleSecuritiesDebt	0	USD
CY2020	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	473000	USD
CY2019	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	605000	USD
CY2018	us-gaap	Payments To Acquire Property Plant And Equipment PaymentsToAcquirePropertyPlantAndEquipment	1370000	USD
CY2020	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	37027000	USD
CY2019	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	-27293000	USD
CY2018	us-gaap	Net Cash Provided By Used In Investing Activities NetCashProvidedByUsedInInvestingActivities	87195000	USD
CY2020	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	3138000	USD
CY2019	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	175000	USD
CY2018	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	918000	USD
CY2020	us-gaap	Proceeds From Issuance Of Shares Under Incentive And Share Based Compensation Plans ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlans	561000	USD
CY2019	us-gaap	Proceeds From Issuance Of Shares Under Incentive And Share Based Compensation Plans ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlans	489000	USD
CY2019	mrsn	Proceeds From Debt Net Issuance Costs ProceedsFromDebtNetIssuanceCosts	4965000	USD
CY2018	mrsn	Proceeds From Debt Net Issuance Costs ProceedsFromDebtNetIssuanceCosts	0	USD
CY2020	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	116000	USD
CY2019	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	87000	USD
CY2018	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	0	USD
CY2020	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	230412000	USD
CY2019	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	97704000	USD
CY2018	us-gaap	Net Cash Provided By Used In Financing Activities NetCashProvidedByUsedInFinancingActivities	1064000	USD
CY2020	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	192743000	USD
CY2019	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	2667000	USD
CY2018	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect	33043000	USD
CY2019Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	62672000	USD
CY2018Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	60005000	USD
CY2017Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	26962000	USD
CY2020Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	255415000	USD
CY2019Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	62672000	USD
CY2018Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	60005000	USD
CY2020	mrsn	Fair Value Of Common Stock Retired In Exchange For Issuance Of Common Stock Warrant FairValueOfCommonStockRetiredInExchangeForIssuanceOfCommonStockWarrant	0	USD
CY2019	mrsn	Fair Value Of Common Stock Retired In Exchange For Issuance Of Common Stock Warrant FairValueOfCommonStockRetiredInExchangeForIssuanceOfCommonStockWarrant	8986000	USD
CY2018	mrsn	Fair Value Of Common Stock Retired In Exchange For Issuance Of Common Stock Warrant FairValueOfCommonStockRetiredInExchangeForIssuanceOfCommonStockWarrant	0	USD
CY2020	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	102000	USD
CY2019	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	0	USD
CY2018	us-gaap	Capital Expenditures Incurred But Not Yet Paid CapitalExpendituresIncurredButNotYetPaid	317000	USD
CY2020	us-gaap	Debt Issuance Costs Incurred During Noncash Or Partial Noncash Transaction DebtIssuanceCostsIncurredDuringNoncashOrPartialNoncashTransaction	0	USD
CY2019	us-gaap	Debt Issuance Costs Incurred During Noncash Or Partial Noncash Transaction DebtIssuanceCostsIncurredDuringNoncashOrPartialNoncashTransaction	180000	USD
CY2018	us-gaap	Debt Issuance Costs Incurred During Noncash Or Partial Noncash Transaction DebtIssuanceCostsIncurredDuringNoncashOrPartialNoncashTransaction	0	USD
CY2020	us-gaap	Interest Paid Net InterestPaidNet	234000	USD
CY2019	us-gaap	Interest Paid Net InterestPaidNet	132000	USD
CY2018	us-gaap	Interest Paid Net InterestPaidNet	0	USD
CY2020	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	9980000	USD
CY2019	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	4369000	USD
CY2018	us-gaap	Right Of Use Asset Obtained In Exchange For Operating Lease Liability RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability	0	USD
CY2020	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	0	USD
CY2019	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	429000	USD
CY2018	us-gaap	Right Of Use Asset Obtained In Exchange For Finance Lease Liability RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability	0	USD
CY2020	mrsn	Adjustment To Retained Earnings Deferred Revenue Upon Adoption Of New Accounting Standard AdjustmentToRetainedEarningsDeferredRevenueUponAdoptionOfNewAccountingStandard	0	USD
CY2019	mrsn	Adjustment To Retained Earnings Deferred Revenue Upon Adoption Of New Accounting Standard AdjustmentToRetainedEarningsDeferredRevenueUponAdoptionOfNewAccountingStandard	0	USD
CY2018	mrsn	Adjustment To Retained Earnings Deferred Revenue Upon Adoption Of New Accounting Standard AdjustmentToRetainedEarningsDeferredRevenueUponAdoptionOfNewAccountingStandard	-2031000	USD
CY2020	us-gaap	Net Income Loss NetIncomeLoss	-88045000	USD
CY2019	us-gaap	Net Income Loss NetIncomeLoss	-28208000	USD
CY2018	us-gaap	Net Income Loss NetIncomeLoss	-64257000	USD
CY2020Q4	us-gaap	Retained Earnings Accumulated Deficit RetainedEarningsAccumulatedDeficit	-280419000	USD
CY2020	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>
CY2020	mrsn	Revenue Collection Period RevenueCollectionPeriod	P30D
CY2019Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	62351000	USD
CY2020Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	255094000	USD
CY2018Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	59634000	USD
CY2019Q4	us-gaap	Cash And Cash Equivalents At Carrying Value CashAndCashEquivalentsAtCarryingValue	62351000	USD
CY2019Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	321000	USD
CY2020Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	321000	USD
CY2018Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	371000	USD
CY2019Q4	us-gaap	Restricted Cash Noncurrent RestrictedCashNoncurrent	321000	USD
CY2019Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	62672000	USD
CY2020Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	255415000	USD
CY2018Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	60005000	USD
CY2019Q4	us-gaap	Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents	62672000	USD
CY2020Q4	us-gaap	Retainage Deposit RetainageDeposit	1832000	USD
CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0
CY2020Q4	us-gaap	Other Assets Noncurrent OtherAssetsNoncurrent	2153000	USD
CY2019Q4	us-gaap	Other Assets Noncurrent OtherAssetsNoncurrent	1453000	USD
CY2019Q4	us-gaap	Retainage Deposit RetainageDeposit	1132000	USD
CY2020Q4	us-gaap	Security Deposit SecurityDeposit	321000	USD
CY2019Q4	us-gaap	Security Deposit SecurityDeposit	321000	USD
CY2020	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	6869189	shares
CY2019	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	5207582	shares
CY2018	us-gaap	Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	3856932	shares
CY2020	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%">The Company has no financial instruments with off-balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. 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 has not experienced any credit losses and does not believe this it is subject to any significant concentrations of credit risk from these financial instruments.</span></div>
CY2020	us-gaap	Concentration Risk Credit Risk Financial Instrument Maximum Exposure ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure	0	USD
CY2019Q4	us-gaap	Contract With Customer Asset Net ContractWithCustomerAssetNet	0	USD
CY2020Q4	us-gaap	Contract With Customer Asset Net ContractWithCustomerAssetNet	0	USD
CY2019Q4	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	4815000	USD
CY2019	us-gaap	Contract With Customer Performance Obligation Satisfied In Previous Period ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod	0	USD
CY2019Q4	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	4815000	USD
CY2020	us-gaap	Contract With Customer Performance Obligation Satisfied In Previous Period ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod	0	USD
CY2020	mrsn	Contract With Customer Liability Increase ContractWithCustomerLiabilityIncrease	0	USD
CY2020	mrsn	Contract With Customer Liability Decrease ContractWithCustomerLiabilityDecrease	828000	USD
CY2020Q4	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	3987000	USD
CY2018Q4	us-gaap	Contract With Customer Asset Net ContractWithCustomerAssetNet	0	USD
CY2019Q4	us-gaap	Contract With Customer Asset Net ContractWithCustomerAssetNet	0	USD
CY2018Q4	us-gaap	Contract With Customer Liability ContractWithCustomerLiability	46196000	USD
CY2019	mrsn	Contract With Customer Liability Increase ContractWithCustomerLiabilityIncrease	210000	USD
CY2019	mrsn	Contract With Customer Liability Decrease ContractWithCustomerLiabilityDecrease	41591000	USD
CY2020	us-gaap	Contract With Customer Liability Revenue Recognized ContractWithCustomerLiabilityRevenueRecognized	828000	USD
CY2019	us-gaap	Contract With Customer Liability Revenue Recognized ContractWithCustomerLiabilityRevenueRecognized	41591000	USD
CY2020Q4	us-gaap	Debt Securities Available For Sale Unrealized Loss Position Number Of Positions DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions	0	security
CY2019Q4	us-gaap	Available For Sale Debt Securities Amortized Cost Basis AvailableForSaleDebtSecuritiesAmortizedCostBasis	37414000	USD
CY2019Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	25000	USD
CY2019Q4	us-gaap	Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	0	USD
CY2019Q4	us-gaap	Available For Sale Securities Debt Securities AvailableForSaleSecuritiesDebtSecurities	37439000	USD
CY2019Q4	us-gaap	Property Plant And Equipment And Finance Lease Right Of Use Asset After Accumulated Depreciation And Amortization PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization	2164000	USD
CY2020	us-gaap	Depreciation Depreciation	1010000	USD
CY2020Q4	us-gaap	Property Plant And Equipment And Finance Lease Right Of Use Asset Before Accumulated Depreciation And Amortization PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization	9365000	USD
CY2019Q4	us-gaap	Property Plant And Equipment And Finance Lease Right Of Use Asset Before Accumulated Depreciation And Amortization PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization	9373000	USD
CY2020Q4	us-gaap	Property Plant And Equipment And Finance Lease Right Of Use Asset Accumulated Depreciation And Amortization PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization	7635000	USD
CY2019Q4	us-gaap	Property Plant And Equipment And Finance Lease Right Of Use Asset Accumulated Depreciation And Amortization PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization	7209000	USD
CY2020Q4	us-gaap	Property Plant And Equipment And Finance Lease Right Of Use Asset After Accumulated Depreciation And Amortization PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization	1730000	USD
CY2019	us-gaap	Depreciation Depreciation	1245000	USD
CY2018	us-gaap	Depreciation Depreciation	1257000	USD
CY2020Q4	us-gaap	Employee Related Liabilities Current EmployeeRelatedLiabilitiesCurrent	5412000	USD
CY2019Q4	mrsn	Accrued Preclinical And Clinical Expenses Current AccruedPreclinicalAndClinicalExpensesCurrent	4230000	USD
CY2019Q4	us-gaap	Employee Related Liabilities Current EmployeeRelatedLiabilitiesCurrent	4037000	USD
CY2020Q4	us-gaap	Accrued Professional Fees Current AccruedProfessionalFeesCurrent	757000	USD
CY2020Q4	mrsn	Accrued Preclinical And Clinical Expenses Current AccruedPreclinicalAndClinicalExpensesCurrent	9902000	USD
CY2019Q4	us-gaap	Accrued Professional Fees Current AccruedProfessionalFeesCurrent	675000	USD
CY2020Q4	us-gaap	Other Accrued Liabilities Current OtherAccruedLiabilitiesCurrent	75000	USD
CY2019Q4	us-gaap	Other Accrued Liabilities Current OtherAccruedLiabilitiesCurrent	44000	USD
CY2020Q4	us-gaap	Accrued Liabilities Current AccruedLiabilitiesCurrent	16146000	USD
CY2019Q4	us-gaap	Accrued Liabilities Current AccruedLiabilitiesCurrent	8986000	USD
CY2020Q4	us-gaap	Long Term Debt Noncurrent LongTermDebtNoncurrent	4977000	USD
CY2020Q4	us-gaap	Debt Instrument Carrying Amount DebtInstrumentCarryingAmount	5200000	USD
CY2020Q4	us-gaap	Long Term Debt Current LongTermDebtCurrent	0	USD
CY2020Q4	mrsn	Debt Instrument Carrying Amount Noncurrent DebtInstrumentCarryingAmountNoncurrent	5200000	USD
CY2020Q4	us-gaap	Deferred Finance Costs Net DeferredFinanceCostsNet	246000	USD
CY2020Q4	us-gaap	Debt Instrument Unamortized Premium DebtInstrumentUnamortizedPremium	23000	USD
CY2020Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Next Twelve Months LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths	0	USD
CY2020Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Two LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo	1213000	USD
CY2020Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Three LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree	2080000	USD
CY2020Q4	us-gaap	Long Term Debt Maturities Repayments Of Principal In Year Four LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour	1907000	USD
CY2020Q4	us-gaap	Debt Instrument Carrying Amount DebtInstrumentCarryingAmount	5200000	USD
CY2020	us-gaap	Interest Expense Debt InterestExpenseDebt	340000	USD
CY2019	us-gaap	Interest Expense Debt InterestExpenseDebt	214000	USD
CY2020Q4	us-gaap	Preferred Stock Shares Authorized PreferredStockSharesAuthorized	25000000	shares
CY2020Q4	us-gaap	Preferred Stock Shares Issued PreferredStockSharesIssued	0	shares
CY2013	mrsn	Class Of Warrant Or Right Securities Called By Warrants Or Rights Number Issued ClassOfWarrantOrRightSecuritiesCalledByWarrantsOrRightsNumberIssued	129491	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
CY2020Q4	us-gaap	Class Of Warrant Or Right Outstanding ClassOfWarrantOrRightOutstanding	39474	shares
CY2019Q4	us-gaap	Class Of Warrant Or Right Outstanding ClassOfWarrantOrRightOutstanding	39474	shares
CY2020	mrsn	Stock Issued During Period Shares Warrants Exercised StockIssuedDuringPeriodSharesWarrantsExercised	0	shares
CY2020Q4	us-gaap	Common Stock Capital Shares Reserved For Future Issuance CommonStockCapitalSharesReservedForFutureIssuance	6869189	shares
CY2019Q4	us-gaap	Common Stock Capital Shares Reserved For Future Issuance CommonStockCapitalSharesReservedForFutureIssuance	7782582	shares
CY2019Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	4720772	shares
CY2019Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	5.24
CY2019	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Remaining Contractual Term2 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2	P7Y3M18D
CY2019Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue	9836000	USD
CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Gross ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross	2359074	shares
CY2020	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Grants In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice	11.96
CY2020	us-gaap	Stock Issued During Period Shares Stock Options Exercised StockIssuedDuringPeriodSharesStockOptionsExercised	697428	shares
CY2020	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Exercises In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice	4.50
CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Expirations In Period ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod	269470	shares
CY2020	us-gaap	Share Based Compensation Arrangements By Share Based Payment Award Options Expirations In Period Weighted Average Exercise Price ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice	7.05
CY2020Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	6112948	shares
CY2020Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	7.84
CY2020	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Remaining Contractual Term2 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2	P7Y3M18D
CY2018	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	918000	USD
CY2020Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Intrinsic Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue	114729000	USD
CY2020Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Number ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber	3147379	shares
CY2020Q4	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Weighted Average Exercise Price ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice	5.14
CY2020	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Exercisable Weighted Average Remaining Contractual Term1 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1	P6Y1M6D
CY2020Q4	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Options Exercisable Intrinsic Value1 SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1	67562000	USD
CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	7.99
CY2019	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	2.47
CY2018	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Grant Date Fair Value ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	8.78
CY2020	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	3138000	USD
CY2019	us-gaap	Proceeds From Stock Options Exercised ProceedsFromStockOptionsExercised	175000	USD
CY2019	us-gaap	Finance Lease Right Of Use Asset Amortization FinanceLeaseRightOfUseAssetAmortization	75000	USD
CY2020	us-gaap	Finance Lease Interest Expense FinanceLeaseInterestExpense	21000	USD
CY2018	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0
CY2020	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Fair Value Assumptions Expected Term1 SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1	P6Y18D
CY2020	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	7172000	USD
CY2019	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	4872000	USD
CY2018	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	3884000	USD
CY2020	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	7172000	USD
CY2019	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	4872000	USD
CY2018	us-gaap	Allocated Share Based Compensation Expense AllocatedShareBasedCompensationExpense	3884000	USD
CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Risk Free Interest Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate	0.012
CY2019	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Risk Free Interest Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate	0.023
CY2018	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Risk Free Interest Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate	0.027
CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0
CY2019	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0
CY2019	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Fair Value Assumptions Expected Term1 SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1	P5Y11M26D
CY2018	us-gaap	Sharebased Compensation Arrangement By Sharebased Payment Award Fair Value Assumptions Expected Term1 SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1	P6Y25D
CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Volatility Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate	0.74
CY2019	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Volatility Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate	0.74
CY2018	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Volatility Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate	0.73
CY2020	us-gaap	Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate	0
CY2020Q1	mrsn	Tenant Improvement Allowance TenantImprovementAllowance	172000	USD
CY2020Q1	mrsn	Lessee Operating Lease Modification Increase Decrease In Right Of Use Assets LesseeOperatingLeaseModificationIncreaseDecreaseInRightOfUseAssets	9980000	USD
CY2020Q1	mrsn	Lessee Operating Lease Modification Increase Decrease In Operating Lease Liability LesseeOperatingLeaseModificationIncreaseDecreaseInOperatingLeaseLiability	9980000	USD
CY2019Q4	us-gaap	Security Deposit SecurityDeposit	321000	USD
CY2020Q4	us-gaap	Security Deposit SecurityDeposit	321000	USD
CY2019Q1	us-gaap	Finance Lease Right Of Use Asset Before Accumulated Amortization FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization	429000	USD
CY2020	us-gaap	Operating Lease Cost OperatingLeaseCost	2755000	USD
CY2019	us-gaap	Operating Lease Cost OperatingLeaseCost	2160000	USD
CY2020	us-gaap	Finance Lease Right Of Use Asset Amortization FinanceLeaseRightOfUseAssetAmortization	176000	USD
CY2020Q4	us-gaap	Finance Lease Liability Noncurrent FinanceLeaseLiabilityNoncurrent	174000	USD
CY2020Q4	us-gaap	Finance Lease Right Of Use Asset Before Accumulated Amortization FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization	429000	USD
CY2019Q4	us-gaap	Finance Lease Right Of Use Asset Before Accumulated Amortization FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization	429000	USD
CY2019	us-gaap	Finance Lease Interest Expense FinanceLeaseInterestExpense	20000	USD
CY2020	mrsn	Finance Lease Cost FinanceLeaseCost	197000	USD
CY2019	mrsn	Finance Lease Cost FinanceLeaseCost	95000	USD
CY2020Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	10936000	USD
CY2019Q4	us-gaap	Operating Lease Right Of Use Asset OperatingLeaseRightOfUseAsset	2598000	USD
CY2020Q4	us-gaap	Operating Lease Liability Current OperatingLeaseLiabilityCurrent	1437000	USD
CY2019Q4	us-gaap	Operating Lease Liability Current OperatingLeaseLiabilityCurrent	2219000	USD
CY2020Q4	us-gaap	Operating Lease Liability Noncurrent OperatingLeaseLiabilityNoncurrent	10158000	USD
CY2019Q4	us-gaap	Operating Lease Liability Noncurrent OperatingLeaseLiabilityNoncurrent	677000	USD
CY2020Q4	us-gaap	Finance Lease Right Of Use Asset Accumulated Amortization FinanceLeaseRightOfUseAssetAccumulatedAmortization	176000	USD
CY2019Q4	us-gaap	Finance Lease Right Of Use Asset Accumulated Amortization FinanceLeaseRightOfUseAssetAccumulatedAmortization	75000	USD
CY2020Q4	us-gaap	Finance Lease Liability Current FinanceLeaseLiabilityCurrent	93000	USD
CY2019Q4	us-gaap	Finance Lease Liability Current FinanceLeaseLiabilityCurrent	87000	USD
CY2019Q4	us-gaap	Finance Lease Liability Noncurrent FinanceLeaseLiabilityNoncurrent	275000	USD
CY2020Q4	us-gaap	Operating Lease Weighted Average Remaining Lease Term1 OperatingLeaseWeightedAverageRemainingLeaseTerm1	P5Y2M12D
CY2019Q4	us-gaap	Operating Lease Weighted Average Remaining Lease Term1 OperatingLeaseWeightedAverageRemainingLeaseTerm1	P1Y3M18D
CY2020Q4	us-gaap	Finance Lease Weighted Average Remaining Lease Term1 FinanceLeaseWeightedAverageRemainingLeaseTerm1	P2Y10M24D
CY2019Q4	us-gaap	Finance Lease Weighted Average Remaining Lease Term1 FinanceLeaseWeightedAverageRemainingLeaseTerm1	P3Y8M12D
CY2020Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.108
CY2019Q4	us-gaap	Operating Lease Weighted Average Discount Rate Percent OperatingLeaseWeightedAverageDiscountRatePercent	0.103
CY2020Q4	us-gaap	Finance Lease Weighted Average Discount Rate Percent FinanceLeaseWeightedAverageDiscountRatePercent	0.069
CY2019Q4	us-gaap	Finance Lease Weighted Average Discount Rate Percent FinanceLeaseWeightedAverageDiscountRatePercent	0.069
CY2020	us-gaap	Operating Lease Payments OperatingLeasePayments	2394000	USD
CY2019	us-gaap	Operating Lease Payments OperatingLeasePayments	2271000	USD
CY2020	us-gaap	Finance Lease Interest Payment On Liability FinanceLeaseInterestPaymentOnLiability	21000	USD
CY2019	us-gaap	Finance Lease Interest Payment On Liability FinanceLeaseInterestPaymentOnLiability	20000	USD
CY2020	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	116000	USD
CY2019	us-gaap	Finance Lease Principal Payments FinanceLeasePrincipalPayments	87000	USD
CY2020	us-gaap	Lease Cost LeaseCost	2644000	USD
CY2019	us-gaap	Lease Cost LeaseCost	2160000	USD
CY2018	us-gaap	Lease Cost LeaseCost	1994000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due Next Twelve Months LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths	2772000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due Next Twelve Months FinanceLeaseLiabilityPaymentsDueNextTwelveMonths	116000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Two LesseeOperatingLeaseLiabilityPaymentsDueYearTwo	2843000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due Year Two FinanceLeaseLiabilityPaymentsDueYearTwo	84000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Three LesseeOperatingLeaseLiabilityPaymentsDueYearThree	2928000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due Year Three FinanceLeaseLiabilityPaymentsDueYearThree	74000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Four LesseeOperatingLeaseLiabilityPaymentsDueYearFour	3016000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due Year Four FinanceLeaseLiabilityPaymentsDueYearFour	18000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due Year Five LesseeOperatingLeaseLiabilityPaymentsDueYearFive	3888000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due Year Five FinanceLeaseLiabilityPaymentsDueYearFive	0	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Payments Due LesseeOperatingLeaseLiabilityPaymentsDue	15447000	USD
CY2020Q4	us-gaap	Finance Lease Liability Payments Due FinanceLeaseLiabilityPaymentsDue	292000	USD
CY2020Q4	us-gaap	Lessee Operating Lease Liability Undiscounted Excess Amount LesseeOperatingLeaseLiabilityUndiscountedExcessAmount	3854000	USD
CY2020Q4	us-gaap	Finance Lease Liability Undiscounted Excess Amount FinanceLeaseLiabilityUndiscountedExcessAmount	25000	USD
CY2020Q4	us-gaap	Operating Lease Liability OperatingLeaseLiability	11593000	USD
CY2020Q4	us-gaap	Finance Lease Liability FinanceLeaseLiability	267000	USD
CY2020	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.210
CY2019	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.210
CY2018	us-gaap	Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	0.210
CY2018	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.065
CY2020	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.067
CY2019	us-gaap	Effective Income Tax Rate Reconciliation State And Local Income Taxes EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	0.061
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Change In Deferred Tax Assets Valuation Allowance EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance	0.179
CY2018	us-gaap	Effective Income Tax Rate Reconciliation Change In Deferred Tax Assets Valuation Allowance EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance	-0.323
CY2018	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0
CY2020	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense EffectiveIncomeTaxRateReconciliationNondeductibleExpense	0.012
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense EffectiveIncomeTaxRateReconciliationNondeductibleExpense	-0.020
CY2018	us-gaap	Effective Income Tax Rate Reconciliation Nondeductible Expense EffectiveIncomeTaxRateReconciliationNondeductibleExpense	0.006
CY2020	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	0.034
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	0.103
CY2018	us-gaap	Effective Income Tax Rate Reconciliation Tax Credits EffectiveIncomeTaxRateReconciliationTaxCredits	0.042
CY2020	us-gaap	Effective Income Tax Rate Reconciliation Other Adjustments EffectiveIncomeTaxRateReconciliationOtherAdjustments	0
CY2019	us-gaap	Effective Income Tax Rate Reconciliation Other Adjustments EffectiveIncomeTaxRateReconciliationOtherAdjustments	-0.533
CY2018	us-gaap	Effective Income Tax Rate Reconciliation Other Adjustments EffectiveIncomeTaxRateReconciliationOtherAdjustments	0
CY2020	us-gaap	Effective Income Tax Rate Reconciliation Change In Deferred Tax Assets Valuation Allowance EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance	-0.323
CY2020	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0
CY2019	us-gaap	Effective Income Tax Rate Continuing Operations EffectiveIncomeTaxRateContinuingOperations	0
CY2020Q4	mrsn	Deferred Tax Assets Lease Liabilities DeferredTaxAssetsLeaseLiabilities	3166000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	64259000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Operating Loss Carryforwards DeferredTaxAssetsOperatingLossCarryforwards	40554000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards DeferredTaxAssetsTaxCreditCarryforwards	5670000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Tax Credit Carryforwards DeferredTaxAssetsTaxCreditCarryforwards	2448000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Accrued Liabilities DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAccruedLiabilities	4058000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Accrued Liabilities DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAccruedLiabilities	2520000	USD
CY2019Q4	mrsn	Deferred Tax Assets Lease Liabilities DeferredTaxAssetsLeaseLiabilities	791000	USD
CY2020Q4	mrsn	Deferred Tax Assets Licensed Technology DeferredTaxAssetsLicensedTechnology	1534000	USD
CY2019Q4	mrsn	Deferred Tax Assets Licensed Technology DeferredTaxAssetsLicensedTechnology	1402000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Deferred Income DeferredTaxAssetsDeferredIncome	1088000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Deferred Income DeferredTaxAssetsDeferredIncome	1315000	USD
CY2020Q4	mrsn	Deferred Tax Assets Depreciation DeferredTaxAssetsDepreciation	502000	USD
CY2020Q4	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	22858000	USD
CY2019Q4	mrsn	Deferred Tax Assets Depreciation DeferredTaxAssetsDepreciation	492000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	84000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Other DeferredTaxAssetsOther	77000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	80361000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Gross DeferredTaxAssetsGross	49599000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	77375000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	48889000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	2986000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Net DeferredTaxAssetsNet	710000	USD
CY2020Q4	us-gaap	Deferred Tax Liabilities Leasing Arrangements DeferredTaxLiabilitiesLeasingArrangements	2986000	USD
CY2019Q4	us-gaap	Deferred Tax Liabilities Leasing Arrangements DeferredTaxLiabilitiesLeasingArrangements	710000	USD
CY2020Q4	us-gaap	Deferred Income Tax Liabilities DeferredIncomeTaxLiabilities	2986000	USD
CY2019Q4	us-gaap	Deferred Income Tax Liabilities DeferredIncomeTaxLiabilities	710000	USD
CY2020Q4	us-gaap	Deferred Tax Assets Liabilities Net DeferredTaxAssetsLiabilitiesNet	0	USD
CY2019Q4	us-gaap	Deferred Tax Assets Liabilities Net DeferredTaxAssetsLiabilitiesNet	0	USD
CY2020Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	77375000	USD
CY2019Q4	us-gaap	Deferred Tax Assets Valuation Allowance DeferredTaxAssetsValuationAllowance	48889000	USD
CY2020	us-gaap	Valuation Allowance Deferred Tax Asset Change In Amount ValuationAllowanceDeferredTaxAssetChangeInAmount	28468000	USD
CY2019	us-gaap	Valuation Allowance Deferred Tax Asset Change In Amount ValuationAllowanceDeferredTaxAssetChangeInAmount	-5051000	USD
CY2019Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	0	USD
CY2020Q4	us-gaap	Unrecognized Tax Benefits UnrecognizedTaxBenefits	0	USD
CY2020Q4	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Accrued UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued	0	USD
CY2019Q4	us-gaap	Unrecognized Tax Benefits Income Tax Penalties And Interest Accrued UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued	0	USD
CY2020Q4	mrsn	Income Taxes Number Of State Jurisdictions IncomeTaxesNumberOfStateJurisdictions	2	jurisdiction
CY2020Q4	mrsn	Income Taxes Number Of Audits In Progress IncomeTaxesNumberOfAuditsInProgress	0	audit
CY2020	mrsn	Defined Contribution Plan Eligible Age To Participate DefinedContributionPlanEligibleAgeToParticipate	P21Y
CY2020	us-gaap	Defined Contribution Plan Maximum Annual Contributions Per Employee Percent DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent	0.75
CY2018	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent Of Match DefinedContributionPlanEmployerMatchingContributionPercentOfMatch	0.50
	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent Of Match DefinedContributionPlanEmployerMatchingContributionPercentOfMatch	0.50
	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent DefinedContributionPlanEmployerMatchingContributionPercent	0.06
CY2018	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent DefinedContributionPlanEmployerMatchingContributionPercent	0.06
	mrsn	Defined Contribution Plan Maximum Annual Employer Contribution Per Employee Amount DefinedContributionPlanMaximumAnnualEmployerContributionPerEmployeeAmount	6000	USD
CY2018	mrsn	Defined Contribution Plan Maximum Annual Employer Contribution Per Employee Amount DefinedContributionPlanMaximumAnnualEmployerContributionPerEmployeeAmount	6000	USD
	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent Of Match DefinedContributionPlanEmployerMatchingContributionPercentOfMatch	1
CY2020Q2	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	15413000	USD
	us-gaap	Defined Contribution Plan Employer Matching Contribution Percent DefinedContributionPlanEmployerMatchingContributionPercent	0.04
	mrsn	Defined Contribution Plan Maximum Annual Employer Contribution Per Employee Amount DefinedContributionPlanMaximumAnnualEmployerContributionPerEmployeeAmount	7000	USD
CY2020	us-gaap	Defined Contribution Plan Cost Recognized DefinedContributionPlanCostRecognized	486000	USD
CY2019	us-gaap	Defined Contribution Plan Cost Recognized DefinedContributionPlanCostRecognized	404000	USD
CY2018	us-gaap	Defined Contribution Plan Cost Recognized DefinedContributionPlanCostRecognized	332000	USD
CY2020Q1	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	11000	USD
CY2020Q2	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	796000	USD
CY2020Q3	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	11000	USD
CY2020Q4	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	10000	USD
CY2020Q1	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	12219000	USD
CY2020Q3	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	16546000	USD
CY2020Q4	us-gaap	Operating Expenses OperatingExpenses	28772000	USD
CY2020Q1	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	4936000	USD
CY2020Q2	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	5171000	USD
CY2020Q3	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	5881000	USD
CY2020Q4	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	5914000	USD
CY2020Q1	us-gaap	Operating Expenses OperatingExpenses	17155000	USD
CY2020Q2	us-gaap	Operating Expenses OperatingExpenses	20584000	USD
CY2020Q3	us-gaap	Operating Expenses OperatingExpenses	22427000	USD
CY2020Q1	us-gaap	Investment Income Interest InvestmentIncomeInterest	306000	USD
CY2020Q2	us-gaap	Investment Income Interest InvestmentIncomeInterest	89000	USD
CY2020Q3	us-gaap	Investment Income Interest InvestmentIncomeInterest	19000	USD
CY2020Q4	us-gaap	Investment Income Interest InvestmentIncomeInterest	10000	USD
CY2020Q1	us-gaap	Interest Expense InterestExpense	88000	USD
CY2020Q2	us-gaap	Interest Expense InterestExpense	87000	USD
CY2020Q3	us-gaap	Interest Expense InterestExpense	92000	USD
CY2020Q4	us-gaap	Interest Expense InterestExpense	92000	USD
CY2020Q1	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	218000	USD
CY2020Q2	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	2000	USD
CY2020Q3	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	-73000	USD
CY2020Q4	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	-82000	USD
CY2020Q1	us-gaap	Net Income Loss NetIncomeLoss	-16926000	USD
CY2020Q2	us-gaap	Net Income Loss NetIncomeLoss	-19786000	USD
CY2020Q3	us-gaap	Net Income Loss NetIncomeLoss	-22489000	USD
CY2020Q4	us-gaap	Net Income Loss NetIncomeLoss	-28844000	USD
CY2020Q1	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-0.35
CY2020Q1	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-0.35
CY2020Q2	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-0.33
CY2020Q2	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-0.33
CY2020Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-0.33
CY2020Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-0.33
CY2020Q4	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-0.42
CY2020Q4	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-0.42
CY2020Q1	us-gaap	Weighted Average Number Of Share Outstanding Basic And Diluted WeightedAverageNumberOfShareOutstandingBasicAndDiluted	47988630	shares
CY2020Q2	us-gaap	Weighted Average Number Of Share Outstanding Basic And Diluted WeightedAverageNumberOfShareOutstandingBasicAndDiluted	60748225	shares
CY2020Q3	us-gaap	Weighted Average Number Of Share Outstanding Basic And Diluted WeightedAverageNumberOfShareOutstandingBasicAndDiluted	68419192	shares
CY2020Q4	us-gaap	Weighted Average Number Of Share Outstanding Basic And Diluted WeightedAverageNumberOfShareOutstandingBasicAndDiluted	68630078	shares
CY2019Q1	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	41035000	USD
CY2019Q2	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	202000	USD
CY2019Q3	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	844000	USD
CY2019Q4	us-gaap	Revenue From Contract With Customer Excluding Assessed Tax RevenueFromContractWithCustomerExcludingAssessedTax	42000	USD
CY2019Q1	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	15143000	USD
CY2019Q2	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	13766000	USD
CY2019Q3	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	13701000	USD
CY2019Q4	us-gaap	Research And Development Expense ResearchAndDevelopmentExpense	12430000	USD
CY2019Q1	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	4443000	USD
CY2019Q2	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	4192000	USD
CY2019Q3	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	4436000	USD
CY2019Q4	us-gaap	General And Administrative Expense GeneralAndAdministrativeExpense	4212000	USD
CY2019Q1	us-gaap	Operating Expenses OperatingExpenses	19586000	USD
CY2019Q2	us-gaap	Operating Expenses OperatingExpenses	17958000	USD
CY2019Q3	us-gaap	Operating Expenses OperatingExpenses	18137000	USD
CY2019Q4	us-gaap	Operating Expenses OperatingExpenses	16642000	USD
CY2019Q1	us-gaap	Investment Income Interest InvestmentIncomeInterest	452000	USD
CY2019Q2	us-gaap	Investment Income Interest InvestmentIncomeInterest	725000	USD
CY2019Q3	us-gaap	Investment Income Interest InvestmentIncomeInterest	608000	USD
CY2019Q4	us-gaap	Investment Income Interest InvestmentIncomeInterest	441000	USD
CY2019Q1	us-gaap	Interest Expense InterestExpense	0	USD
CY2019Q2	us-gaap	Interest Expense InterestExpense	40000	USD
CY2019Q3	us-gaap	Interest Expense InterestExpense	107000	USD
CY2019Q4	us-gaap	Interest Expense InterestExpense	87000	USD
CY2019Q1	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	452000	USD
CY2019Q2	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	685000	USD
CY2019Q3	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	501000	USD
CY2019Q4	us-gaap	Nonoperating Income Expense NonoperatingIncomeExpense	354000	USD
CY2019Q1	us-gaap	Net Income Loss NetIncomeLoss	21901000	USD
CY2019Q2	us-gaap	Net Income Loss NetIncomeLoss	-17071000	USD
CY2019Q3	us-gaap	Net Income Loss NetIncomeLoss	-16792000	USD
CY2019Q4	us-gaap	Net Income Loss NetIncomeLoss	-16246000	USD
CY2019Q1	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	0.72
CY2019Q2	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-0.36
CY2019Q3	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-0.35
CY2019Q4	us-gaap	Earnings Per Share Basic EarningsPerShareBasic	-0.34
CY2019Q1	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	0.70
CY2019Q2	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-0.36
CY2019Q3	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-0.35
CY2019Q4	us-gaap	Earnings Per Share Diluted EarningsPerShareDiluted	-0.34
CY2019Q1	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	30299650	shares
CY2019Q2	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	47708085	shares
CY2019Q3	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	47833607	shares
CY2019Q4	us-gaap	Weighted Average Number Of Shares Outstanding Basic WeightedAverageNumberOfSharesOutstandingBasic	47886144	shares
CY2019Q1	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	31461696	shares
CY2019Q2	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	47708085	shares
CY2019Q3	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	47833607	shares
CY2019Q4	us-gaap	Weighted Average Number Of Diluted Shares Outstanding WeightedAverageNumberOfDilutedSharesOutstanding	47886144	shares

Files In Submission

Name	View Source	Status
0001628280-21-003370-index-headers.html	Edgar Link	pending
0001628280-21-003370-index.html	Edgar Link	pending
0001628280-21-003370.txt	Edgar Link	pending
0001628280-21-003370-xbrl.zip	Edgar Link	pending
exhibit21110kfy2020q4.htm	Edgar Link	pending
exhibit23110kfy2020q4.htm	Edgar Link	pending
exhibit31110kfy2020q4.htm	Edgar Link	pending
exhibit31210kfy2020q4.htm	Edgar Link	pending
exhibit32110kfy2020q4.htm	Edgar Link	pending
FilingSummary.xml	Edgar Link	unprocessable
Financial_Report.xlsx	Edgar Link	pending
MetaLinks.json	Edgar Link	pending
mrsn-20201231.htm	Edgar Link	pending
mrsn-20201231.xsd	Edgar Link	pending
mrsn-20201231_cal.xml	Edgar Link	unprocessable
mrsn-20201231_def.xml	Edgar Link	unprocessable
mrsn-20201231_g1.jpg	Edgar Link	pending
mrsn-20201231_g10.jpg	Edgar Link	pending
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mrsn-20201231_g7.jpg	Edgar Link	pending
mrsn-20201231_g8.jpg	Edgar Link	pending
mrsn-20201231_g9.jpg	Edgar Link	pending
mrsn-20201231_htm.xml	Edgar Link	completed
mrsn-20201231_lab.xml	Edgar Link	unprocessable
mrsn-20201231_pre.xml	Edgar Link	unprocessable
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report.css	Edgar Link	pending
Show.js	Edgar Link	pending

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$MRSN
Mersana Therapeutics, Inc. Stock $mrsn

2020 Q4 Form 10-K Financial Statement

#000162828021003370 Filed on February 26, 2021

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$MRSN Mersana Therapeutics, Inc. Stock $mrsn

2020 Q4 Form 10-K Financial Statement

#000162828021003370 Filed on February 26, 2021

Income Statement

Balance Sheet

Cashflow Statement

Facts In Submission

Files In Submission

$MRSN
Mersana Therapeutics, Inc. Stock $mrsn