Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=124260329&loc=d3e26626-111562

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of liability recognized arising from contingent consideration in a business combination, expected to be settled beyond one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 25
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 35
-Paragraph 1
-Subparagraph b
-URI http://asc.fasb.org/extlink&oid=116859824&loc=d3e6819-128478

+ Details

Name:	us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=CLSN_InprocessRandDMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($)	Jun. 30, 2021	Dec. 31, 2020
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investment securities, available for sale	$ 31,923,285
Earn-out milestone liability	7,088,000	7,018,000
Fair Value, Recurring [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Earn-out milestone liability	7,088,000	7,018,000
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Earn-out milestone liability
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Earn-out milestone liability
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Earn-out milestone liability	7,088,000	7,018,000
Fair Value, Recurring [Member] \| Corporate Debt Securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investment securities, available for sale	31,923,285
Fair Value, Recurring [Member] \| Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 1 [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investment securities, available for sale
Fair Value, Recurring [Member] \| Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investment securities, available for sale	31,923,285
Fair Value, Recurring [Member] \| Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investment securities, available for sale
Fair Value, Nonrecurring [Member] \| Inprocess R&D [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investment securities, available for sale	13,366,234	13,366,234
Fair Value, Nonrecurring [Member] \| Inprocess R&D [Member] \| Fair Value, Inputs, Level 1 [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investment securities, available for sale
Fair Value, Nonrecurring [Member] \| Inprocess R&D [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investment securities, available for sale
Fair Value, Nonrecurring [Member] \| Inprocess R&D [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investment securities, available for sale	$ 13,366,234	$ 13,366,234