Significant Accounting Policies (Policies)	12 Months Ended
Significant Accounting Policies (Policies)	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Accounting, Policy [Policy Text Block]	Basis of Presentation The accompanying consolidated financial statements of Celsion have been prepared in accordance with generally accepted accounting principles (“GAAP”) in the United States and include the accounts of the Company and CLSN Laboratories, Inc. All intercompany balances and transactions have been eliminated. The preparation of financial statements in conformity with GAAP requires management to make judgments, estimates, and assumptions that affect the amount reported in the Company’s financial statements and accompanying notes. Actual results could differ materially from these estimates. Events and conditions arising subsequent to the most recent balance sheet date through the date of the issuance of these consolidated financial statements have been evaluated for their possible impact on the financial statements and accompanying notes. No events and conditions would give rise to any information that required accounting recognition or disclosure in the financial statements other than those arising in the ordinary course of business.
Revenue Recognition, Policy [Policy Text Block]	Revenue Recognition At the inception of each collaborative agreement that includes milestone payments, the Company evaluates whether each milestone is substantive on the basis of the contingent nature of the milestone, specifically reviewing factors such as the scientific and other risks that must be overcome to achieve the milestone, as well as the level of effort and investment required. Milestones that are not considered substantive and that do not meet the separation criteria are accounted for as license payments and recognized on a straight-line basis over the remaining period of performance. Payments received or reasonably assured after performance obligations are met completely are recognized as earned.
Cash and Cash Equivalents, Policy [Policy Text Block]	Cash and Cash Equivalents Cash and cash equivalents include cash on hand and investments purchased with an original maturity of three months or less. A portion of these funds are not covered by FDIC insurance.
Fair Value of Financial Instruments, Policy [Policy Text Block]	Fair Value of Investment Securities The carrying values of investment securities approximate their respective fair values.
Investment, Policy [Policy Text Block]	Short Term Investments The Company classifies its investments in marketable securities with readily determinable fair values as investments available-for-sale in accordance with Accounting Standards Codification (ASC) 320, Investments - Debt and Equity Securities . Available-for-sale securities consist of debt and equity securities not classified as trading securities or as securities to be held to maturity. The Company has classified all of its investments as available-for-sale. Unrealized holding gains and losses on available-for-sale securities are reported as a net amount in accumulated other comprehensive gain or loss in stockholders’ equity until realized. Gains and losses on the sale of available-for-sale securities are determined using the specific identification method. The Company’s short-term investments consist of corporate bonds and government agency bonds.
Property, Plant and Equipment, Policy [Policy Text Block]	Property and Equipment Property and equipment is stated at cost less accumulated depreciation and amortization. Depreciation is provided over the estimated useful lives of the related assets, ranging from three to seven years, using the straight-line method. Amortization is recognized over the lesser of the life of the asset or the lease term. Major renewals and improvements are capitalized at cost and ordinary repairs and maintenance are charged against operating expenses as incurred. Depreciation expense was approximately $ 326,000 and $455,000 for the years ended December 31, 2017 and 2016, respectively. The Company reviews property and equipment for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. An asset is considered impaired if its carrying amount exceeds the future net undiscounted cash flows that the asset is expected to generate. If such asset is considered to be impaired, the impairment recognized is the amount by which the carrying amount of the asset, if any, exceeds its fair value determined using a discounted cash flow model.
Deposits [Policy Text Block]	Deposits Deposits include real property security deposits and other deposits which are contractually required and of a long-term nature.
Goodwill and Intangible Assets, Intangible Assets, Policy [Policy Text Block]	In-Process Research and Development, Other Intangible Assets and Goodwill During 2014, the Company acquired certain assets of EGEN, Inc. As more fully described in Note 5, the acquisition was accounted for under the acquisition method of accounting which required the Company to perform an allocation of the purchase price to the assets acquired and liabilities assumed. Under the acquisition method of accounting, the total purchase price is allocated to net tangible and intangible assets and liabilities based on their estimated fair values as of the acquisition date. Patent Licenses The Company has purchased several licenses for rights to patented technologies. Patent license costs of $75,000 have been capitalized and are amortized on a straight-line basis over the estimated life of the related patent. As of December 31, 2017 and 2016, the total accumulated amortization expense is $75,00 . The weighted-average amortization period for these assets is 10 years.
Comprehensive Income, Policy [Policy Text Block]	Comprehensive Income (Loss) Accounting Standards Codification (“ ASC”) 220, Comprehensive Income , establishes standards for the reporting and display of comprehensive income (loss) and its components in the Company’s consolidated financial statements. The objective of ASC 220 is to report a measure comprehensive income (loss) of all changes in equity of an enterprise that result from transactions and other economic events in a period other than transactions with owners.
Research and Development Expense, Policy [Policy Text Block]	Research and Development Research and development costs are expensed as incurred. Equipment and facilities acquired for research and development activities that have alternative future uses are capitalized and charged to expense over their estimated useful lives.
Earnings Per Share, Policy [Policy Text Block]	Net Loss Per Common Share Basic and diluted net loss per common share was computed by dividing net loss for the year by the weighted average number of shares of common stock outstanding, both basic and diluted, during each period. The impact of common stock equivalents has been excluded from the computation of diluted weighted average common shares outstanding in periods where there is a net loss, as their effect is anti-dilutive. For the year ended December 31, 2017, the total number of shares of common stock issuable upon exercise of warrants and equity awards is 3,761,844. For the year ended December 31, 2017, diluted loss per common share is the same as basic loss per common share as all options and all warrants that were convertible into shares of the Company’s common stock were excluded from the calculation of diluted earnings attributable to common stockholders per common share as their effect would be anti-dilutive. For the year ended December 31, 2016, the total number of shares of common stock issuable upon exercise of warrants and equity awards was 1,702,272. The Pre-funded Series B Warrants (as more fully described in Note 10 of these financial statements) were convertible into shares of the Company’s common stock totaling 132,142 were considered issued in calculating basic loss per share. For the year ended December 31, 2016, diluted loss per common share was the same as basic loss per common share as the other 1,570,300 warrants and equity awards that were convertible into shares of the Company’s common stock were excluded from the calculation of diluted earnings per common share as their effect would have been anti-dilutive.
Income Tax, Policy [Policy Text Block]	Income Taxes Income taxes are accounted for under the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carry forwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax asset and liabilities of a change in tax rates is recognized in results of operations in the period that the tax rate change occurs. Valuation allowances are established, when necessary, to reduce deferred tax assets to the amount expected to be realized. In accordance with ASC 740, Income Taxes, a tax position is recognized as a benefit only if it is “more likely than not” that the tax position taken would be sustained in a tax examination, presuming that a tax examination will occur. The Company recognizes interest and/or penalties related to income tax matters in the income tax expense category.
Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block]	Stock-Based Compensation In March 2016, the FASB issued ASU 2016 - 09, Compensation - Stock Compensation , which simplifies various aspects of accounting for share-based payments. The areas for simplification involve several aspects of the accounting for share-based payment transactions, including the income tax consequences and classification on the statements of cash flows. The Company adopted the standard during the first quarter of 2017 and has elected to recognize the effect of forfeitures in compensation cost when they occur. There was no retrospective impact to the consolidated financial statements, including the consolidated statements of cash flows as a result of the adoption of this standard.
Reclassification, Policy [Policy Text Block]	Reclassifications Certain reclassifications have been made to prior year financial statements to conform to classifications used in the current year. These classifications had no impact on net loss, stockholders ’ equity or cash flows as previously reported. See Note 10 regarding the reverse stock split which occurred on May 26, 2017.
New Accounting Pronouncements, Policy [Policy Text Block]	Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“FASB”) and are adopted by us as of the specified effective date. Unless otherwise discussed, we believe that the impact of recently issued accounting pronouncements will not have a material impact on the Company ’s consolidated financial position, results of operations, and cash flows, or do not apply to our operations. In May 2014, the FASB issued Accounting Standards Update (ASU) No. 2014 - 09 “Revenue from Contracts with Customers (Topic 606 ),” which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. ASU 2014 - 09 was originally going to be effective on January 1, 2017; however, the FASB issued ASU 2015 - 14, “Revenue from Contracts with Customers (Topic 606 ) — Deferral of the Effective Date,” which deferred the effective date of ASU 2014 - 09 by one year to January 1, 2018. In March 2016, the FASB issued ASU No. 2016 - 8, “Revenue from Contracts with Customers (Topic 606 ): Principal versus Agent Considerations. The amendments in this ASU do not change the core principle of ASU No. 2014 - 09 but the amendments clarify the implementation guidance on reporting revenue gross versus net. The effective date for the amendments in this ASU is the same as the effective date of ASU No. 2014 - 09. In April 2016, the FASB issued ASU No. 2016 - 10, “Revenue from Contracts with Customers (Identifying Performance Obligations and Licensing),” to clarify the implementation guidance on identifying performance obligations and licensing. The standard allows for either “full retrospective” adoption, meaning the standard is applied to all of the periods presented, or “modified retrospective” adoption, meaning the standard is applied only to the most current period presented in the financial statements. The Company will adopt the standard on January 1, 2018 using the modified retrospective approach. The Company conducted its contract review process in 2017 to make appropriate changes to business and control processes to support recognition and disclosure under the new standard. Based on the Company’s evaluation, the adoption of the ASU 2014 - 09 will not have a material impact on its consolidated financial statements because existing contractual performance obligations, which determine when and how revenue is recognized, are not materially changed under the new standard. In January 2016, the FASB issued Accounting Standards Update No. 2016 - 01, Recognition and Measurement of Financial Assets and Financial Liabilities, which requires that most equity investments be measured at fair value, with subsequent changes in fair value recognized in net income (other than those accounted for under the equity method of accounting). This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. Based on the Company’s evaluation to date, the adoption of the ASU 2016 - 01 is not expected to have a material impact on its consolidated financial statements or its disclosures. In February 2016, the FASB issued Accounting Standards Update No. 2016 - 02, Leases (Topic 842 ), which requires that lessees recognize assets and liabilities for leases with lease terms greater than twelve months in the statement of financial position. Leases will be classified as either finance or operating, with classification affecting the pattern of expense recognition in the income statement. This update also requires improved disclosures to help users of financial statements better understand the amount, timing and uncertainty of cash flows arising from leases. The update is effective for fiscal years beginning after December 15, 2018, including interim reporting periods within that reporting period. Early adoption is permitted. The Company is currently evaluating the impact the adoption of this guidance will have on its consolidated financial statements and disclosures. In August 2016, the FASB issued Accounting Standard Update No. 2016 - 15, Statement of Cash Flows (Topic 230 ): Classification of Certain Cash Receipts and Cash Payments. This update clarifies how certain cash receipts and payments should be presented in the statement of cash flows and is effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. Based on the Company’s evaluation to date, the adoption of the ASU 2016 - 15 is not expected to have a material impact on its consolidated financial statements or its disclosures. In November 2016, the FASB issued Accounting Standard Update No. 2016 - 18, Statement of Cash Flows (Topic 230 ): Restricted Cash. This update amends the guidance in ASC 230, including providing additional guidance related to transfers between cash and restricted cash and how entities present, in their statement of cash flows, the cash receipts and cash payments that directly affect the restricted cash accounts. This guidance is effective for annual reporting periods beginning after December 15, 2017, and interim periods within those years, with early adoption permitted. Based on the Company’s evaluation to date, the adoption of the ASU 2016 - 18 is not expected to have a material impact on its consolidated financial statements or its disclosures. In January 2017, the FASB issued Accounting Standard Update No. 2017 - 01, Business Combinations (Topic 805 ): Clarifying the Definition of a Business, which clarifies the definition of a business with the objective of adding guidance to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. This guidance is effective for annual reporting periods beginning after December 15, 201 7, and interim periods within those years, with early adoption permitted. Based on the Company’s evaluation, the adoption of the ASU 2016 - 18 is not expected to have a material impact on its consolidated financial statements or its disclosures. In January 2017, the FASB issued Accounting Standard Update No. 2017 - 04, Intangibles-Goodwill and Other, Simplifying the Test for Goodwill impairment, which eliminates Step 2 from the goodwill impairment test. Under the revised test, an entity should perform its annual, or interim, goodwill impairment test by comparing the fair value of a reporting unit with its carrying amount. An entity should recognize an impairment charge for the amount by which the carrying amount exceeds the reporting unit's fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. This ASU is effective for any interim or annual impairment tests for fiscal years beginning after December 15, 2019, with early adoption permitted. The Company adopted this method for its impairment test of goodwill during 2017.