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Earn-out Milestone Liability - Schedule of Changes in Earn-out Milestone Liability (Details)	3 Months Ended
	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