Document And Entity Information Note To Financial Statement Details Textual us-gaap_AmortizationOfFinancingCosts Amortization of Debt Issuance Costs statementnote6investmentsecuritiesavailableforsaletables statementnote7fairvalueofmeasurementstables statementnote8acquisitionofegeninctables statementnote9accruedliabilitiestables Short-term investments, fair value statementnote10notepayabletables us-gaap_InterestExpenseDebt Interest Expense, Debt statementnote12stockbasedcompensationtables statementnote13earnoutmilestoneliabilitytables statementnote14warrantstables Basis of Accounting [Text Block] statementnote6investmentsecuritiesavailableforsaleshortterminvestmentbybondmaturitiesdetails statementnote6investmentsecuritiesavailableforsaleinvestmentincomedetails statementnote6investmentsecuritiesavailableforsaleinvestmentsecuritiesgrossunrealizedlossesandfairvaluebyinvestmentcategorydetails statementnote6investmentsecuritiesavailableforsalecomponentofaccumulatedothercomprehensivelossdetails us-gaap_IncreaseDecreaseInDeferredRevenue Deferred revenue statementnote7fairvalueofmeasurementsassetsandliabilitiesatfairvalueonarecurringbasisdetails statementnote8aquisitionofegenincsummaryofestimatedfairvaluesofacquiredassetsandliabilitiesdetails statementnote9accruedliabilitiesaccruedliabilitiesdetails statementnote10notepayablenotespayablewithherculescreditagreementdetails Short Term Investments Available for Sale [Member] Information relating to short term investments classified as available for sale. statementnote10notepayablefutureprincipalpaymentsdueonthecreditagreementdetails statementnote12stockbasedcompensationstockoptionandrestrictedstockawardsdetails Accrued liabilities statementnote12stockbasedcompensationfairvalueofoptionassumptionsdetails statementnote13earnoutmilestoneliabilitysummaryofthechangesintheearnoutmilestoneliabilitydetails Certificate of Deposit [Member] Savings certificate entitling the bearer to receive interest. statementnote13earnoutmilestoneliabilityscheduleofmilestoneriskadjustmentassessmentdetails statementnote14warrantssummaryofwarrantactivitydetails Notes To Financial Statements us-gaap_AreaOfRealEstateProperty Area of Real Estate Property Notes To Financial Statements [Abstract] us-gaap_LesseeLeasingArrangementsOperatingLeasesTermOfContract Lessee Leasing Arrangements, Operating Leases, Term of Contract Investment Maturing Between 3 and 12 Months [Member] Maturity of short-term investment. Investment Maturing Within Three Months [Member] Maturity of investment. us-gaap_PolicyTextBlockAbstract Accounting Policies us-gaap_IncreaseDecreaseInAccountsPayable Accounts payable clsn_AvailableForSaleSecuritiesWithContinuousGainPositionLessThan12MonthsFairValue Short-term investments with unrealized gains Available for sale securities with continuous gain position less than 12 months fair value. us-gaap_ComprehensiveIncomeNetOfTax Comprehensive loss clsn_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedGain Short-term investments with unrealized gains Amount of accumulated unrealized gains on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months. 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. Operating expenses: Preferred stock, shares outstanding (in shares) Common stock, shares outstanding (in shares) us-gaap_IncreaseDecreaseInDeferredLiabilities Change in deferred rent liability Cash and cash equivalents Cash and cash equivalents at beginning of period Cash and cash equivalents at end of period Nature of Operations [Text Block] us-gaap_AllocatedShareBasedCompensationExpense Allocated Share-based Compensation Expense us-gaap_IncreaseDecreaseInOtherOperatingAssets Increase (Decrease) in Other Operating Assets Change in fair value of earn-out milestone liability Security deposit on letter of credit us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets Advances, deposits and other current assets Other assets: us-gaap_IncreaseDecreaseInDepositOtherAssets Accrued interest on short term investments Accrued interest receivable on investment securities us-gaap_TreasuryStockValue Treasury stock, at cost (22,920 and 75,924 shares at September 30, 2016 and December 31, 2015, respectively) us-gaap_StockIssuedDuringPeriodSharesAcquisitions Stock Issued During Period, Shares, Acquisitions Licensing revenue Fair Value Disclosures [Text Block] 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. 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) Sale of Stock [Axis] Sale of Stock [Domain] us-gaap_StockIssuedDuringPeriodSharesNewIssues Stock Issued During Period, Shares, New Issues us-gaap_LiabilitiesAndStockholdersEquity Total liabilities and stockholders’ equity Accumulated deficit Retained Earnings (Accumulated Deficit) Accumulated other comprehensive income (loss) Balance at January 1, 2016 Balance at September 30, 2016 Accrued interest on notes payable Accrued interest on notes payable. Hercules [Member] Represents Hercules Technology Growth Capital, Inc.. Class of Warrant or Right [Domain] Subtotal other assets clsn_InterestOnlyPaymentsPeriod Interest Only Payments Period Represents the time period in which payments are interest only. Class of Warrant or Right [Axis] us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1 Class of Warrant or Right, Exercise Price of Warrants or Rights clsn_AmortizationPeriodForPrincipalAndInterest Amortization Period for Principal and Interest The period for amortization of principal and interest. us-gaap_ClassOfWarrantOrRightOutstanding Class of Warrant or Right, Outstanding Warrants outstanding (in shares) Warrants outstanding (in shares) us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight Class of Warrant or Right, Number of Securities Called by Each Warrant or Right 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. us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights Class of Warrant or Right, Number of Securities Called by Warrants or Rights Statement of Comprehensive Income [Abstract] Patent licensing fees and other assets, net clsn_ClassificationOfWarrantsToFromCommonStockWarrantLiabilityNet Classification of Warrants to (from) Common Stock Warrant Liability Net Amount of classification of warrants to (from) common stock warrant liability after tax. 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_FairValueAdjustmentOfWarrants Change in fair value of common stock warrant liability Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Stock-based compensation costs Available for Sale (all unrealized holding gains and losses are less than 12 months at date of measurement) 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. Treasury stock, shares (in shares) Schedule of Milestone Risk Adjustment Assessment [Table Text Block] Tabular disclosure of the risk-adjustment assessment of each milestone. Common stock - $0.01 par value (112,500,000 shares authorized; 26,083,493 and 23,395,211 shares issued at September 30, 2016 and December 31, 2015, respectively and 26,060,573 and 23,319,287 shares outstanding at September 30, 2016 and December 31, 2015, respectively) us-gaap_TableTextBlock Notes Tables May 2015 Common Stock Offering [Member] Represents a securities purchase agreement with certain investors into which the entity entered in May 2015. Accretion of investment premium 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). Common stock, shares issued (in shares) Common stock, shares authorized (in shares) Amendment Flag us-gaap_DepreciationDepletionAndAmortization Depreciation and amortization Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share clsn_CommonStockOfferingPeriodofrestriction Common Stock Offering PeriodOfRestriction Represents period after the closing of common stock offering during which the Company is prohibited from effecting or entering into an agreement to issue common stock or any other securities that are at any time convertible into, or exercisable or exchangeable for, or otherwise entitle the holder thereof to receive, common stock to the extent such issuance or sale involves certain variable conversion, exercise or exchange prices or such agreement provides for sale of securities at a price to be determined in the future. 2018 and thereafter us-gaap_AssetsCurrent Subtotal current assets 2017 us-gaap_InterestAndDebtExpense Interest expense Other income (expense) us-gaap_NonoperatingIncomeExpense Total other (expense) income, net Preferred Stock - $0.01 par value (100,000 shares authorized and no shares issued or outstanding at September 30, 2016 and December 31, 2015, respectively) 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 Preferred stock, shares issued (in shares) Amounts due to contract research organizations and other contractual agreements Contractual Obligation Series A Warrant [Member] Series A Convertible Preferred Stock (Member) Statement of Financial Position [Abstract] Preferred stock, shares authorized (in shares) 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 Interest and dividends accrued and paid Document Fiscal Year Focus Preferred stock, par value (in dollars per share) Document Period End Date Series B Warrant [Member] Information relating to Series B warrants. Investment income, net Investment income, net Document Type Statement of Cash Flows [Abstract] us-gaap_OperatingIncomeLoss Loss from operations Document Information [Line Items] Document Information [Table] Interest paid clsn_CurrentPeriodReclassificationAdjustment Current Period Reclassification Adjustment The amount of a reclassification adjustment made to current period financial statement amounts.) Entity Filer Category Entity Current Reporting Status December 31, 2015 [Member] Represents the reporting period of 2015. us-gaap_PaymentsForRent Payments for Rent Entity Voluntary Filers Reclassification from Deferred Financing Fees to Deduction from Debt Liability [Member] Represents the reclassification of deferred financing fess. Entity Well-known Seasoned Issuer Entity Central Index Key Entity Registrant Name Entity [Domain] Business Acquisition, Acquiree [Domain] Legal Entity [Axis] Business Acquisition [Axis] Additional paid-in capital us-gaap_ProceedsFromLicenseFeesReceived Proceeds from License Fees Received us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High Stockholders’ equity: Stock Incentive Plan 2007 [Member] Represents Celsion Corporation 2007 Stock Incentive Plan that was adopted in 2007. Entity Common Stock, Shares Outstanding (in shares) Cash flows from operating activities: Statement [Line Items] Expected forfeiture rate The estimated forfeiture rate of share-based payment awards. us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued Business Acquisition, Equity Interest Issued or Issuable, Number of Shares Trading Symbol Earn-out Milestone Liability Disclosure [Text Block] The disclosure related to earn-out milestone liabilities. us-gaap_Liabilities Total liabilities Risk-adjustment Assessment Risk Adjusted Assessment of Each Milestone The percentage of risk-adjusted assessment of each milestone. Commitments and contingencies Investment securities – available for sale, at fair value Short-term Investments Fair Value, Inputs, Level 2 [Member] Fair Value, Inputs, Level 3 [Member] Estimated Time to Achieve Estimated Time to Achieve the Milestone The amount of estimated time to achieve the milestone. Fair Value Hierarchy [Domain] Fair Value, Inputs, Level 1 [Member] Fair Value, Hierarchy [Axis] clsn_GainLossFromChangeInFairValueOfEarnOutMilestoneLiability Gain (Loss) from Change in Fair Value of Earn out Milestone Liability Non-cash charge from the adjustment for the change in fair value included in net loss The amount of gain (loss) from the change in the fair value of earn-out milestone liability. Current assets: us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease Decrease in cash and cash equivalents clsn_EarnoutMilestoneLiability Balance Balance The amount of earn-out milestone liability as of the balance sheet. 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. us-gaap_NetCashProvidedByUsedInFinancingActivities Net cash provided by financing activities Type of Adoption [Domain] Adjustments for New Accounting Pronouncements [Axis] us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis Short-term investments maturities Schedule of Warrant Activity [Table Text Block] Tabular disclosure of warrant activity. us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet Net assets acquired us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities Accounts payable and accrued liabilities Warrants issued during the nine months ended September 30, 2016 (see Note 11) (in shares) The number of warrants or rights issued during the period. clsn_ClassOfWarrantOrRightExpiredDuringPeriod Warrants expired during the nine months ended September 30, 2016 (in shares) The number of warrant or right that expired during the period. clsn_ClassOfWarrantOrRightWeightedAverageRemainingContractualTerms Weighted average remaining contractual terms Represents weighted average remaining contractual terms of warrants. Short-term investments, cost us-gaap_StockholdersEquity Total stockholders’ equity us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets Total assets: Total Total us-gaap_ProceedsFromIssuanceOfCommonStock Proceeds from Issuance of Common Stock Property and equipment, net us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss Short-term investments with unrealized losses us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax Realized loss reclassified from other accumulated comprehensive loss Unrealized gain reclassified to other accumulated income Common Stock [Member] Equity Components [Axis] us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles In-process research and development Equity Component [Domain] Report Date [Axis] Financial Statement Filing Date [Domain] us-gaap_DeferredRevenue Deferred Revenue Expected dividend yield Risk-free interest rate us-gaap_LongTermDebt Total Proceeds from sale of common stock equity, net of issuance costs Expected volatility us-gaap_BusinessCombinationConsiderationTransferred1 Business Combination, Consideration Transferred Total convertible notes payable Expected life (in years) us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred Business Combination, Consideration Transferred, Liabilities Incurred Short-term investments with unrealized losses Aggregate intrinsic value of outstanding awards, non-vested restricted stock Aggregate intrinsic value of outstanding awards, options Other comprehensive gain Other comprehensive gain, net 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) Equity awards granted, non-vested restricted stock weighted average grant date fair value (in dollars per share) Equity awards exercised, 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) us-gaap_DeferredFinanceCostsNet Debt Issuance Costs, Net Unamortized debt issuance costs Accounting Policies [Abstract] Equity awards granted, non-vested restricted stock (in shares) Lawrenceville New Jersey [Member] Property located in Lawrenceville New Jersey. Equity awards exercisable, contractual term us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableAggregateIntrinsicValue Aggregate intrinsic value of awards exercisable, options Equity awards exercisable, weighted average exercise price (in dollars per share) us-gaap_DebtInstrumentUnamortizedDiscount Debt Instrument, Unamortized Discount Equity awards exercisable, options (in shares) clsn_NumberOfMonthsInWhichFreeRentIsProvided Number of Months in Which Free Rent Is Provided The number of months in which free rent is provided. (Loss) gain from change in valuation of earn-out milestone liability The amount recognized in earnings related to the change in fair value during the period of milestone liability. us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value Equity awards outstanding, contractual term Credit Facility [Domain] clsn_ContractualObligationReductionFortyThreeMonthsFromInitialTerm Contractual Obligation Reduction Forty Three Months from Initial Term Represents the reduction of the contractual obligation that will occur forty-three months from the initial term. clsn_EscrowDepositRelatedToOperatingLease Escrow Deposit Related to Operating Lease Escrow deposit related to premises under operating lease obligations. Series C Warrant [Member] Represents information about Series C Warrant. clsn_ContractualObligationReductionThirtyOneMonthsFromInitialTerm Contractual Obligation Reduction Thirty One Months from Initial Term Represents the reduction of the contractual obligation that will occur thirty-one months from the initial term. June 2016 Common Stock Offering [Member] The Securities Purchase Agreement classified as "June 2016 Purchase Agreement" made the Company with an investor. Credit Facility [Axis] 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_OwnershipPercentageRestrictionThresholdForWarrantExercise Ownership Percentage, Restriction Threshold for Warrant Exercise Represents the ownership restriction threshold for a holder to exercise warrant. 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) clsn_NumberOfDaysNoticeRequirementToIncreaseOrDecreaseBeneficialOwnershipLimitation Number of Days Notice Requirement to Increase or Decrease Beneficial Ownership Limitation Represents the number of days notice requirement to increase or decrease beneficial ownership limitation. 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. clsn_ClassOfWarrantOrRightAggregateIntrinsicValueOfOutstandingWarrants Aggregate intrinsic value of outstanding warrants Represents the aggregate intrinsic value of outstanding warrants. 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) Legal Entity Type of Counterparty [Domain] Legal Entity of Counterparty, Type [Axis] Maximum [Member] Advances, deposits and other current assets Minimum [Member] Range [Axis] us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised Equity awards exercised, non-vested restricted stock (in shares) Range [Domain] 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. us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber Equity awards outstanding, options (in shares) Equity awards outstanding, options (in shares) clsn_DeferredRevenueAmortizationPeriod Deferred Revenue Amortization Period The amortization period for deferred revenue. Hisun [Member] Represents Zhejiang Hisun Pharmaceutical Co. Ltd. us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod Equity awards forfeited, cancelled or expired, options (in shares) Lease Arrangement, Type [Domain] Lease Arrangement, Type [Axis] us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Corporate Debt Securities [Member] us-gaap_RepaymentsOfNotesPayable Principal payments on notes payable us-gaap_RepaymentsOfLinesOfCredit Repayments of Lines of Credit us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Domain] 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. us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Available-for-sale Securities [Table Text Block] us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period us-gaap_ProceedsFromLinesOfCredit Proceeds from Lines of Credit Accounts Payable and Accrued Liabilities Disclosure [Text Block] In Process Research and Development [Member] Plan Name [Axis] Plan Name [Domain] Basic and diluted (in shares) us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Weighted average shares outstanding us-gaap_SharePrice Share Price us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1 Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Basic and diluted (in dollars per share) Award Type [Axis] Equity Award [Domain] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [Domain] Contingent Consideration by Type [Axis] us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount Contingent Consideration Type [Domain] Scenario, Unspecified [Domain] Scenario [Axis] clsn_CommonStockWarrantLiability Common Stock Warrant Liability Carrying value as of the balance sheet date of common stock warrant liability. Loss from change in valuation of common stock warrant liability Gain (Loss) From Change In Valuation Of Common Stock Warrant Liability The amount recognized in earnings related to the change in fair value during the period of common stock warrant liability. Disclosure of Compensation Related Costs, Share-based Payments [Text Block] clsn_TreasuryStockContributedTo401KPlan Shares issued out of treasury Fair value of shares issued from treasury stock for 401K matching contributions. Warrants [Text Block] The entire disclosure for warrant liabilities. Schedule of Accrued Liabilities [Table Text Block] Statement [Table] Commitments and Contingencies Disclosure [Text Block] Earn-out milestone liability Other liabilities – non-current Income Statement [Abstract] Cash flows from financing activities: us-gaap_GoodwillAcquiredDuringPeriod Goodwill, Acquired During Period us-gaap_NetCashProvidedByUsedInInvestingActivities Net cash provided by investing activities Business Combination Disclosure [Text Block] Description of New Accounting Pronouncements Not yet Adopted [Text Block] Schedule of Unrealized Loss on Investments [Table Text Block] Deferred revenue – non-current portion Investment Type [Axis] Investments [Domain] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] us-gaap_PaymentsToAcquireBusinessesGross Payments to Acquire Businesses, Gross Notes payable – non-current portion us-gaap_DeferredFinanceCostsGross Debt Issuance Costs, Gross Amortized end of term charges net of amortized debt discounts us-gaap_LiabilitiesCurrent Total current liabilities us-gaap_FinitelivedIntangibleAssetsAcquired1 Finite-lived Intangible Assets Acquired Class of Stock [Axis] Realized gains Proceeds from sale and maturity of investment securities Deferred revenue – current portion Investment Income [Table Text Block] Counterparty Name [Domain] Counterparty Name [Axis] Notes payable - current portion Convertible Debt [Table Text Block] Schedule of Maturities of Long-term Debt [Table Text Block] us-gaap_GainLossOnSaleOfSecuritiesNet Loss realized on sale of investment securities Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] us-gaap_PaymentsToAcquireAvailableForSaleSecurities Purchases of investment securities us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 Debt Instrument, Basis Spread on Variable Rate us-gaap_FairValueAssumptionsExpectedVolatilityRate Fair Value Assumptions, Expected Volatility Rate us-gaap_OperatingExpenses Total operating expenses General and administrative Discount Rate Principal amount Substantial Doubt about Going Concern [Text Block] Accrued payroll and related benefits Accounts payable - trade Other accrued liabilities Total Supplemental disclosures of cash flow information: Debt Instrument [Axis] us-gaap_LegalFees Legal Fees Debt Instrument, Name [Domain] Other Accrued professional fees us-gaap_PaymentsToAcquirePropertyPlantAndEquipment Purchases of property and equipment Standby Letters of Credit [Member] Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block] Current liabilities: us-gaap_Assets Total assets us-gaap_WeightedAverageNumberOfSharesContingentlyIssuable Weighted Average Number of Shares, Contingently Issuable us-gaap_StockholdersEquityBeforeTreasuryStock Subtotal Stockholders' Equity Note Disclosure [Text Block] us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity Line of Credit Facility, Maximum Borrowing Capacity us-gaap_FiniteLivedIntangibleAssetsNet In-process research and development us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic Net loss Net loss Goodwill Accumulated depreciation Restricted Stock [Member] Property and equipment (at cost, less accumulated depreciation and amortization of $2,418,483 and $2,058,483, respectively) Employee Stock Option [Member] Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Debt Disclosure [Text Block] clsn_ClassOfWarrantsOrRightExercisedDuringPeriod Warrants exercised during the nine months ended September 30, 2016 (see Note 11) (in shares) The number of warrants or rights exercised during the period. us-gaap_FairValueAssumptionsRiskFreeInterestRate Fair Value Assumptions, Risk Free Interest Rate us-gaap_DisclosureTextBlockAbstract Notes to Financial Statements us-gaap_FairValueAssumptionsExpectedTerm Fair Value Assumptions, Expected Term Other (expense) income: Fair Value, Measurements, Recurring [Member] clsn_StockIssuedDuringPeriodSharesExerciseOfWarrants Stock Issued During Period, Shares, Exercise of Warrants The number of new stock issued with the exercise of some warrants during the period. Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Earnings Per Share [Text Block] us-gaap_NetCashProvidedByUsedInOperatingActivities Net cash (used in) operating activities: us-gaap_PriorPeriodReclassificationAdjustment Prior Period Reclassification Adjustment Cash flows from investing activities: Research and development clsn_DeferredFinanceCharges Amortization of deferred finance charges and debt discount associated with notes payable Deferred finance charges. Refund of security deposit on letter of credit The cash inflow from the refund of deposit for a letter of credit. us-gaap_AvailableForSaleSecuritiesDebtSecurities Short-term investments maturities