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Earn-out Milestone Liability - Schedule of Changes in Earn-out Milestone Liability (Details) | 3 Months Ended
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Mar. 31, 2021 USD ($)
Hercules Warrant [Member] |
Earn-out liabilities, beginning balance | $ (7,018,000)
Non-cash gain loss from the adjustment for the change in fair value | (151,000)
Earn-out liabilities, ending balance | $ (7,169,000)

X
- Definition Earn-out liabilities, beginning balance. + References No definition available. + Details
Name: | CLSN_EarnoutMilestoneLiability
Namespace Prefix: | CLSN_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | CLSN_EarnoutMilestoneLiabilityAbstract
Namespace Prefix: | CLSN_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of gain (loss) from the change in the fair value of earn-out milestone liability. + References No definition available. + Details
Name: | CLSN_GainLossFromChangeInFairValueOfEarnOutMilestoneLiability
Namespace Prefix: | CLSN_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration