Document And Entity Information Note To Financial Statement Details Textual statementnote6fairvalueofmeasurementstables statementnote7acquisitionofegenassetstables statementnote8accruedliabilitiestables statementnote11stockbasedcompensationtables statementnote12earnoutmilestoneliabilitytables statementnote13warrantstables statementnote6fairvalueofmeasurementsassetsandliabilitiesatfairvalueonarecurringbasisdetails Operating expenses: statementnote7acquisitionofegenassetssummaryofestimatedfairvaluesofacquiredassetsandliabilitiesdetails statementnote8accruedliabilitiesotheraccruedliabilitiesdetails statementnote11stockbasedcompensationstockoptionandrestrictedstockdetails statementnote11stockbasedcompensationfairvalueofoptionassumptionsdetails statementnote12earnoutmilestoneliabilitysummaryofthechangesintheearnoutmilestoneliabilitydetails statementnote12earnoutmilestoneliabilityscheduleofmilestoneriskadjustmentassessmentdetails statementnote13warrantssummaryofwarrantactivitydetails Notes To Financial Statements Notes To Financial Statements [Abstract] Short-term investments, fair value Pre-Funded Series BB Warrant [Member] Information relating to Pre-Funded Series BB Warrant. Series AA Warrant [Member] Information relating to Series AA warrants. clsn_ClassOfWarrantOrRightOfferedPriceOfWarrantsOrRights Class of Warrant or Right, Offered Price of Warrants or Rights Offered price per share or per unit of warrants or rights. Cash flows from investing activities: Lease Arrangement, Type [Axis] Lease Arrangement, Type [Domain] clsn_AggregateOfferingPriceAdditions Aggregate Offering Price, Additions Represents the additional aggregate offering price for shares of common stock that the Company may offer and sell through a sales agent. us-gaap_IncreaseDecreaseInDeferredRevenue Deferred revenue us-gaap_ComprehensiveIncomeNetOfTax Comprehensive loss Accrued liabilities Preferred stock, shares outstanding (in shares) Common stock, shares outstanding (in shares) Common Stock, Shares, Outstanding us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity Line of Credit Facility, Maximum Borrowing Capacity us-gaap_IncreaseDecreaseInAccountsPayable Accounts payable Substantial Doubt about Going Concern [Text Block] Subsequent Event [Member] Basis of Accounting [Text Block] Subsequent Event Type [Domain] Subsequent Event Type [Axis] Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Cash and cash equivalents at beginning of period Cash and cash equivalents at end of period Subsequent Events [Text Block] us-gaap_AreaOfRealEstateProperty Area of Real Estate Property us-gaap_IncreaseDecreaseInDeferredLiabilities Change in deferred rent liability Debt Disclosure [Text Block] Licensing revenue Discount rate us-gaap_TreasuryStockValue Treasury stock, at cost (334 shares at September 30, 2017 and December 31, 2016) us-gaap_StockholdersEquityNoteStockSplitConversionRatio1 Stockholders' Equity Note, Stock Split, Conversion Ratio us-gaap_IncreaseDecreaseInOtherOperatingAssets Change in fair value of earn-out milestone liability us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets Advances, deposits and other current assets us-gaap_IncreaseDecreaseInDepositOtherAssets Accrued interest on short term investments us-gaap_StockholdersEquityBeforeTreasuryStock Subtotal Accrued interest receivable on investment securities Stockholders' Equity Note Disclosure [Text Block] us-gaap_SharesIssuedPricePerShare Shares Issued, Price Per Share us-gaap_StockIssuedDuringPeriodSharesAcquisitions Stock Issued During Period, Shares, Acquisitions us-gaap_AllocatedShareBasedCompensationExpense Allocated Share-based Compensation Expense EGEN Inc [Member] Information pertaining to the business acquisition of EGEN, Inc. Ovarian Cancer Study Milestone [Member] Earnout payments that will become payable upon achieving certain specified development milestones relating to an EGEN-001 ovarian cancer study to be conducted by Celsion or its subsidiary. TheraSilence Technology Milestone [Member] Earnout payments that will become payable upon achieving certain specified development milestones relating to the TheraSilence technology acquired from EGEN. Nature of Operations [Text Block] Glioblastoma Multiforme Brain Cancer Study Milestone [Member] Earnout payments that will become payable upon achieving certain specified development milestones relating to an EGEN-001 glioblastoma multiforme brain cancer study to be conducted by Celsion or its subsidiary. Hercules Credit Agreement [Member] Represents a loan agreement with Hercules Technology Growth Capital, Inc. (“Hercules”). Equity awards granted, options (in shares) us-gaap_StockIssuedDuringPeriodSharesNewIssues Stock Issued During Period, Shares, New Issues us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationTransfersIntoLevel3 Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers into Level 3 Accrued interest on notes payable Accrued interest on notes payable. us-gaap_LiabilitiesAndStockholdersEquity Total liabilities and stockholders’ equity Accumulated deficit Retained Earnings (Accumulated Deficit) Hercules [Member] Represents Hercules Technology Growth Capital, Inc.. clsn_AmortizationPeriodForPrincipalAndInterest Amortization Period for Principal and Interest The period for amortization of principal and interest. First Fifty Percent [Member] Represents first 50% of a warrant issued. Hercules Warrant [Member] Represents Hercules Technology Growth Capital, Inc.. Second Fifty Percent [Member] Represents second fifty percent of a warrant issued. Security deposit on letter of credit clsn_DeemedDividendRelatedToWarrantModification Deemed dividend related to warrant modification Dividends deemed that are related to warrant modifications. us-gaap_DisclosureTextBlockAbstract Notes to Financial Statements clsn_LineOfCreditFacilityOriginalLoanFeePercent Line of Credit Facility Original Loan Fee Percent The percent associated with the fees incurred on borrowings from lines of credit. us-gaap_AssetsNoncurrent Subtotal other assets Proceeds from exercise of common stock warrants, net of issuance costs The cash inflow associated with the amount received from holders exercising their stock warrants, net of issuance costs. Other assets: us-gaap_InterestAndDebtExpense Interest expense us-gaap_NonoperatingIncomeExpense Total other expense, net Fair Value, Measurements, Recurring [Member] Other income Fair Value, Measurement Frequency [Domain] Measurement Frequency [Axis] Net changes in: us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount Fair Value, Assets, Level 1 to Level 2 Transfers, Amount Patent licensing fees and other assets, net clsn_WarrantsIssuedExpirationPeriod Warrants Issued, Expiration Period The number of years after which warrants issued are set to expire. clsn_ShelfRegistrationStatementAmount Shelf Registration Statement Amount Shelf Registration statement. clsn_NumberOfWarrantsSoldWithEachShareOfCommonStock Number of Warrants Sold with Each Share of Common Stock The number of warrants sold with each share of common stock per the underwriting agreement. Investment income, net October 2017 Undwriting Offering [Member] The underwriting warrant and share purchase agreement entered into during October of 2017. Underwriter [Member] Represents the underwriter for the underwriting offering. clsn_ProceedsFromIssuanceOfCommonStockAndWarrantExercises Proceeds from Issuance of Common Stock and Warrant Exercises The cash inflow from the additional capital contribution to the entity and the amount received from holders exercising their stock warrants. Schedule of Milestone Risk Adjustment Assessment [Table Text Block] Tabular disclosure of the risk-adjustment assessment of each milestone. us-gaap_OperatingIncomeLoss Loss from operations May 2015 Common Stock Offering [Member] Represents a securities purchase agreement with certain investors into which the entity entered in May 2015. clsn_ProceedsFromIssuanceOfCommonStockGross Proceeds from Issuance of Common Stock, Gross The cash inflow from the additional capital contribution to the entity before the deduction of costs related to the issuance (for example, the placement agent fee and offering expenses). Treasury stock, shares (in shares) Amendment Flag Common stock - $0.01 par value (112,500,000 shares authorized; 5,914,560 and 2,230,452 shares issued at June 30, 2017 and December 31, 2016, respectively, and 5,914,226 and 2,230,118 shares outstanding at June 30, 2017 and December 31, 2016, respectively) Investor Warrants [Member] Warrants offered with common stock from the underwriting agreement in October. Reverse Stock Split [Member] The conversion of a reverse stock split where there is a reduction in the shares outstanding. clsn_ProceedsFromIssuanceOfCommonStockAndWarrants Proceeds from Issuance of Common Stock and Warrants The cash inflow from the additional capital contribution to the entity and from issuance of rights to purchase common shares at predetermined price (usually issued together with corporate debt). Common stock, shares authorized (in shares) Common Stock, Shares Authorized us-gaap_LesseeLeasingArrangementsOperatingLeasesTermOfContract Lessee, Operating Lease, Term of Contract Common stock, shares issued (in shares) Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share February 2013 Preferred Stock Offering [Member] Represents Securities Purchase Agreement with certain institutional investors, into which the Company entered in February 2013. Stock-based compensation costs ATM Agreement [Member] Represents Controlled Equity Offering Sales Agreement (the “ATM Agreement”) with Cantor Fitzgerald & Co., as sales agent. Current Fiscal Year End Date clsn_CommissionOnProceedsFromAtmSharesPercent Commission on Proceeds from ATM Shares Percent Commission on Proceeds from ATM Shares, Percent Series A Warrant [Member] Series A Convertible Preferred Stock (Member) clsn_AggregateOfferingPrice Aggregate Offering Price Represents maximum aggregate offering price for shares of common stock that the Company may offer and sell through a sales agent. Document Fiscal Period Focus Document Fiscal Year Focus January 2014 Common Stock Offering [Member] Represents Securities Purchase Agreement with certain institutional investors into which the entity entered in January 2014. Document Period End Date Preferred Stock - $0.01 par value (100,000 shares authorized and no shares issued or outstanding at September 30, 2017 and December 31, 2016, respectively) us-gaap_FiniteLivedIntangibleAssetUsefulLife Finite-Lived Intangible Asset, Useful Life clsn_IncreaseDecreaseInFairValueOfWarrants Increase (Decrease) in Fair Value of Warrants The amount of increase (decrease) in the fair value of warrants exercised. Preferred stock, shares issued (in shares) Document Type us-gaap_DepreciationDepletionAndAmortization Depreciation and amortization Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Document Information [Line Items] Document Information [Table] Preferred stock, par value (in dollars per share) us-gaap_AssetsCurrent Subtotal current assets Entity Filer Category Entity Current Reporting Status Entity Voluntary Filers Entity Well-known Seasoned Issuer Series AAA Warrants [Member] Represents the information pertaining to the Series AAA Warrants. Series BBB Warrants [Member] Represents the information pertaining to the Series BBB Warrants. July 2017 Direct Offering [Member] Represents the information pertaining to the direct offering agreement on July 6, 2017. Pre-Funded Series CCC Warrants [Member] Represents the information pertaining to the Pre-Funded Series CCC Warrants. Non-cash items included in net loss: 2013 Warrants, 2014 Warrants, and June 2016 Warrants [Member] Represents the information pertaining to the 2013 Warrants, 2014 Warrants, and June 2016 Warrants. Entity Central Index Key clsn_BeneficialOwnershipPercentageThreshold Beneficial Ownership Percentage Threshold Beneficial ownership percentage threshold for purchase of shares of company's common stock. Entity Registrant Name 2015 Warrants [Member] Represents the information pertaining to the 2015 Warrants. Entity [Domain] Series DDD Warrants [Member] Represents the information pertaining to Series DDD Warrants. Legal Entity [Axis] Class of Warrant or Right [Domain] Series AAA and Series BBB Warrants [Member] Represents the information pertaining to the Series AAA and Series BBB Warrants. clsn_ClassOfWarrantOrRightExercisablePeriod Class of Warrant or Right, Exercisable Period The period in which the warrants or rights are exercisable. October 2017 Exercise Agreements [Member] Represents the information pertaining to the letter agreements, classified as the Exercise Agreements, with the holders of Series AAA and Series BBB Warrants issued in the July 2017 Direct Offering. us-gaap_ClassOfWarrantOrRightOutstanding Class of Warrant or Right, Outstanding Warrants outstanding (in shares) Warrants outstanding (in shares) Class of Warrant or Right [Axis] clsn_StockIssuedDuringPeriodSharesAdditionalExerciseOfWarrants Stock Issued During Period, Shares, Additional Exercise of Warrants The additional number of new stock issued with the exercise of warrants during period. us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1 Class of Warrant or Right, Exercise Price of Warrants or Rights Warrants outstanding (in dollars per share) Warrants outstanding (in dollars per share) us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight Class of Warrant or Right, Number of Securities Called by Each Warrant or Right us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights Class of Warrant or Right, Number of Securities Called by Warrants or Rights Stock Incentive Plan 2007 [Member] Represents Celsion Corporation 2007 Stock Incentive Plan that was adopted in 2007. Entity Common Stock, Shares Outstanding (in shares) Interest paid us-gaap_PaymentsForRent Payments for Rent Additional paid-in capital Stockholders’ equity: Expected forfeiture rate The estimated forfeiture rate of share-based payment awards. Trading Symbol Earn-out Milestone Liability Disclosure [Text Block] The disclosure related to earn-out milestone liabilities. Risk-adjustment assessment Risk Adjusted Assessment of Each Milestone The percentage of risk-adjusted assessment of each milestone. Other comprehensive (loss) gain us-gaap_StockholdersEquity Total stockholders’ equity Estimated time to achieve (Year) Estimated Time to Achieve the Milestone The amount of estimated time to achieve the milestone. Non-cash benefit from the adjustment for the change in fair value included in net loss Gain (Loss) from Change in Fair Value of Earn out Milestone Liability The amount of gain (loss) from the change in the fair value of earn-out milestone liability. Commitments and contingencies us-gaap_Liabilities Total liabilities clsn_ClassOfWarrantOrRightExercisedDuringPeriod Class of Warrant or Right, Exercised During Period The number of warrants or rights exercised during period. us-gaap_ProceedsFromLicenseFeesReceived Proceeds from License Fees Received clsn_EarnoutMilestoneLiability Balance Balance The amount of earn-out milestone liability as of the balance sheet. Cash flows from operating activities: Statement [Line Items] Schedule of Changes in Earn out Milestone Liabilities [Table Text Block] The disclosure of transactions regarding the changes in earn-out liabilities during the period. Investment securities – available for sale, at fair value us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax Realized loss on investment securities recognized in investment income, net Unrealized gain on investment securities Fair Value Disclosures [Text Block] Current assets: Schedule of Warrant Activity [Table Text Block] Tabular disclosure of warrant activity. Warrants issued during the nine months ended September 30, 2017 (see Note 10) (in shares) Class of Warrant or Right, Issued During Period The number of warrants or rights issued during the period. clsn_ClassOfWarrantOrRightIssuedDuringPeriodWeightedAverageExercisePrice Warrants issued during the nine months ended September 30, 2017 (see Note 10) (in dollars per share) The weighted average exercise price of warrants that have been issued during the period. clsn_ClassOfWarrantOrRightWeightedAverageRemainingContractualTerms Weighted average remaining contractual terms (Year) Represents weighted average remaining contractual terms of warrants. us-gaap_NetCashProvidedByUsedInFinancingActivities Net cash provided by financing activities Amounts due to contract research organizations and other contractual agreements Contractual Obligation us-gaap_NetCashProvidedByUsedInInvestingActivities Net cash provided by investing activities us-gaap_NetCashProvidedByUsedInOperatingActivities Net cash (used in) operating activities: Sale of Stock [Domain] us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease Increase (decrease) in cash and cash equivalents Sale of Stock [Axis] us-gaap_OtherComprehensiveIncomeLossNetOfTax Other comprehensive gain Indefinite-lived in Process Research and Development [Member] Indefinite-lived in process investigation of new knowledge useful in developing new product or service or new process or technique or improvement to existing product or process, and translation of knowledge into plan or design for new product or process or for improvement to existing product or process. Business Acquisition, Acquiree [Domain] Business Acquisition [Axis] us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High us-gaap_DeferredRevenue Deferred Revenue February 14, 2017 Public Offering [Member] Represents the public offering on February 14, 2017. us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued Business Acquisition, Equity Interest Issued or Issuable, Number of Shares clsn_LesseeLeasingArrangementsOperatingLeaseAdditionalTermOfContract Lessee, Operating Lease, Additional Term of Contract Additional term of lessee's extended operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. us-gaap_ProceedsFromIssuanceOfWarrants Proceeds from Issuance of Warrants Fair Value, Inputs, Level 1 [Member] us-gaap_ProceedsFromWarrantExercises Proceeds from Warrant Exercises Fair Value, Inputs, Level 3 [Member] Lawrenceville New Jersey [Member] Information pertaining to property located in Lawrenceville, New Jersey. Fair Value Hierarchy [Domain] Fair Value, Hierarchy [Axis] us-gaap_ProceedsFromIssuanceOfCommonStock Proceeds from Issuance of Common Stock us-gaap_DeferredFinanceCostsNet Debt Issuance Costs, Net Equity Component [Domain] Equity Components [Axis] clsn_NumberOfMonthsInWhichFreeRentIsProvided Number of Months in Which Free Rent Is Provided The number of months in which free rent is provided. Proceeds from sale of common stock equity, net of issuance costs us-gaap_DebtInstrumentUnamortizedDiscount Debt Instrument, Unamortized Discount Series C Warrant [Member] Represents information about Series C Warrant. June 2016 Common Stock Offering [Member] The Securities Purchase Agreement classified as "June 2016 Purchase Agreement" made the Company with an investor. clsn_ContractualObligationRemaining Contractual Obligation Remaining The remaining amount due under contractual obligations. Series D Warrant [Member] Represents information about Series D Warrant. clsn_ContractualObligationReductionNineteenMonthsFromInitialTerm Contractual Obligation Reduction Nineteen Months from Initial Term Represents the reduction of the contractual obligation that will occur nineteen months from initial term. clsn_OperatingLeasesMonthlyRentPaymentInFirstYear Operating Leases, Monthly Rent Payment in First Year Represents the payment to be paid each month in the first year under an operating lease. clsn_OperatingLeasesMonthlyRentPaymentInFinalYear Operating Leases, Monthly Rent Payment in Final Year Represents the payment to be paid each month in the final year under an operating lease. clsn_ClassOfWarrantOrRightTerm Class of Warrant or Right, Term Period of time the warrants or rights are issued, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. clsn_MaximumBeneficialOwnershipOfThePurchaser Maximum Beneficial Ownership of the Purchaser Represents the maximum beneficial ownership of the Purchaser. clsn_ReductionOfEscrowDeposit Reduction of Escrow Deposit Represents the reduction of the escrow deposit. Aggregate intrinsic value of outstanding warrants Represents the aggregate intrinsic value of outstanding warrants. Basic and diluted (in shares) Basic and diluted (in dollars per share) us-gaap_SharePrice Share Price us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet Net assets acquired us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities Accounts payable and accrued liabilities Weighted average shares outstanding Premises Located in Huntsville Alabama [Member] Represents information about the premises located in Huntsville, Alabama. clsn_NumberOfSharesOfCommonStockIssuableUponExerciseOfWarrantsAndEquityAwards Number of Shares of Common Stock Issuable Upon Exercise of Warrants and Equity Awards Represents the total number of Shares of common stock issuable upon exercise of warrants and equity awards. Licensing Transaction [Text Block] The entire disclosure for a contractual arrangement, in which the Company gave another party an exclusive license. clsn_WarrantsExercisablePeriod Warrants Exercisable Period Represents the period in which warrants are initially exercisable. clsn_OperatingLeasesMonthlyRentPayment Operating Leases, Monthly Rent Payment Represents the payment to be paid each month under an operating lease. clsn_DeferredRevenueAmortizationPeriod Deferred Revenue Amortization Period The amortization period for deferred revenue. Hisun [Member] Represents Zhejiang Hisun Pharmaceutical Co. Ltd. Net loss per common share us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets Total assets Property and equipment, net Scenario, Unspecified [Domain] Scenario [Axis] Schedule of Share-based Compensation Stock Options and Restricted Stock Awards Activity [Table Text Block] The tabular disclosure for share-based compensation, stock options and restricted stock awards, activity. Nonmonetary Transaction Type [Domain] us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles Other intangible assets (Covenant not to compete) Nonmonetary Transaction Type [Axis] us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets In-process research and development Schedule of Accrued Liabilities [Table Text Block] us-gaap_BusinessCombinationConsiderationTransferred1 Business Combination, Consideration Transferred Maximum [Member] Range [Domain] Other income (expense): us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred Business Combination, Consideration Transferred, Liabilities Incurred Minimum [Member] Range [Axis] Accounting Policies [Abstract] Statement of Financial Position [Abstract] us-gaap_BusinessCombinationContingentConsiderationLiability Business Combination, Contingent Consideration, Liability clsn_TreasuryStockContributedTo401KPlan Shares issued out of treasury Fair value of shares issued from treasury stock for 401K matching contributions. Statement of Cash Flows [Abstract] Warrants [Text Block] The entire disclosure for warrant liabilities. us-gaap_RepaymentsOfNotesPayable Principal payment and end of term charges on note payable us-gaap_RepaymentsOfLinesOfCredit Repayments of Lines of Credit us-gaap_ProceedsFromLinesOfCredit Proceeds from Lines of Credit Legal Entity Type of Counterparty [Domain] Legal Entity of Counterparty, Type [Axis] Earn-out milestone liability Other liabilities – non-current portion Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Indefinite-lived Intangible Assets, Major Class Name [Domain] Indefinite-lived Intangible Assets [Axis] us-gaap_PolicyTextBlockAbstract Accounting Policies Accounts Payable and Accrued Liabilities Disclosure [Text Block] Credit Facility [Domain] Statement [Table] Noncompete Agreements [Member] Credit Facility [Axis] Deferred revenue – non-current portion Income Statement [Abstract] Cash flows from financing activities: Class of Stock [Axis] Award Type [Axis] Equity Award [Domain] Description of New Accounting Pronouncements Not yet Adopted [Text Block] Finite-Lived Intangible Assets, Major Class Name [Domain] Contingent Consideration Type [Domain] Finite-Lived Intangible Assets by Major Class [Axis] Contingent Consideration by Type [Axis] us-gaap_GainLossOnSaleOfSecuritiesNet Loss realized on sale of investment securities us-gaap_LiabilitiesCurrent Subtotal current liabilities us-gaap_PaymentsToAcquireBusinessesGross Payments to Acquire Businesses, Gross us-gaap_OperatingExpenses Total operating expenses Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] General and administrative Expected dividend yield us-gaap_GoodwillAcquiredDuringPeriod Goodwill, Acquired During Period Risk-free interest rate Expected volatility Deferred revenue – current portion Business Combination Disclosure [Text Block] Expected life (Year) us-gaap_LegalFees Legal Fees Aggregate intrinsic value of outstanding awards, non-vested restricted stock Commitments and Contingencies Disclosure [Text Block] Equity awards forfeited, cancelled or expired, non-vested restricted stock weighted average grant date fair value (in dollars per share) Notes payable - current portion us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue Equity awards outstanding, non-vested restricted stock weighted average grant date fair value (in dollars per share) Equity awards outstanding, non-vested restricted stock weighted average grant date fair value (in dollars per share) us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber Equity awards outstanding, non-vested restricted stock (in shares) Equity awards outstanding, non-vested restricted stock (in shares) Proceeds from sale and maturity of investment securities Equity awards granted, non-vested restricted stock weighted average grant date fair value (in dollars per share) us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod Equity awards granted, non-vested restricted stock (in shares) Equity awards exercisable, contractual term (Year) Aggregate intrinsic value of awards exercisable, options Advances, deposits and other current assets us-gaap_AssetImpairmentCharges Asset Impairment Charges Equity awards exercisable, weighted average exercise price (in dollars per share) Equity awards exercisable, options (in shares) Equity awards outstanding, contractual term (Year) us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value Accounts payable - trade Accrued payroll and related benefits Other accrued liabilities Total Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block] Impairment of in-process research and development Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill) Loss from impairment of in-process research and development Counterparty Name [Domain] Counterparty Name [Axis] us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice Equity awards outstanding, weighted average exercise price (in dollars per share) Equity awards outstanding, weighted average exercise price (in dollars per share) us-gaap_IndefinitelivedIntangibleAssetsAcquired Indefinite-lived Intangible Assets Acquired us-gaap_PaymentsToAcquireAvailableForSaleSecurities Purchases of investment securities Equity awards forfeited, cancelled or expired, weighted average exercise price (in dollars per share) Equity awards granted, weighted average exercise price (in dollars per share) Other us-gaap_AmortizationOfIntangibleAssets Amortization of Intangible Assets Accrued professional fees us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsForfeituresAndExpirations Equity awards forfeited, cancelled or expired, non-vested restricted stock (in shares) us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Equity awards outstanding, options (in shares) Equity awards outstanding, options (in shares) us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod Equity awards forfeited, cancelled or expired, options (in shares) Current liabilities: us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic Net loss attributable to common shareholders us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant us-gaap_DeferredFinanceCostsGross Debt Issuance Costs, Gross us-gaap_Assets Total assets us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period us-gaap_PaymentsToAcquirePropertyPlantAndEquipment Purchases of property and equipment Net loss Net loss Plan Name [Axis] Plan Name [Domain] us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1 Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Standby Letters of Credit [Member] Restricted Stock [Member] Other intangible assets, net Finite-Lived Intangible Assets, Net clsn_AggregateOfCommonAndPreferredStockSharesAuthorized Aggregate of Common and Preferred Stock, Shares Authorized The aggregate maximum number of common shares and nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws. us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization Finite-Lived Intangible Assets, Accumulated Amortization Employee Stock Option [Member] December 2016 Common Stock Offering [Member] The Securities Purchase Agreement classified as "December 2016 Purchase Agreement" made by the company with an investor. Research and development Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Statement of Comprehensive Income [Abstract] Supplemental disclosures of cash flow information: In-process research and development us-gaap_AmortizationOfFinancingCosts Amortization of Debt Issuance Costs Goodwill Property and equipment (at cost, less accumulated depreciation and amortization of $2,854,122 and $2,513,022, respectively) us-gaap_InterestExpenseDebt Interest Expense, Debt Property and equipment, accumulated depreciation Debt Instrument [Axis] clsn_ClassOfWarrantsOrRightExercisedDuringPeriodWeightedAverageExercisePrice Warrants exercised during the nine months ended September 30, 2017 (see Note 10) (in dollars per share) The weighted average exercise price of the warrants or rights that are exercised during the period. Debt Instrument, Name [Domain] clsn_ClassOfWarrantsOrRightExercisedDuringPeriod Warrants exercised during the nine months ended September 30, 2017 (see Note 10) (in shares) The number of warrants or rights exercised during the period. us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 Debt Instrument, Basis Spread on Variable Rate us-gaap_TableTextBlock Notes Tables Gain (loss) from change in valuation of earn-out milestone liability clsn_GainLossFromValuationOfEarnoutMilestoneLiability The amount recognized in earnings related to the change in fair value during the period of milestone liability. clsn_StockIssuedDuringPeriodSharesExerciseOfWarrants Stock Issued During Period, Shares, Exercise of Warrants The number of new stock issued with the exercise of warrants during the period. Amortization of deferred finance charges and debt discount associated with notes payable Deferred finance charges. Refund of security for letter of credit The cash inflow from the refund of deposit for a letter of credit. Earnings Per Share [Text Block]