Earn-out Milestone Liability - Schedule of Changes in Earn-out Milestone Liability (Details)	6 Months Ended
Jun. 30, 2018 USD ($)
Earn-out Milestone Liability
Earn-out liabilities, beginning balance	$ 12,538,525
Non-cash charge from the adjustment for the change in fair value included in net loss	547,324
Earn-out liabilities, ending balance	$ 13,085,849

The amount of gain (loss) from the change in the fair value of earn-out milestone liability.

No definition available.

Name:	CLSN_GainLossFromChangeInFairValueOfEarnOutMilestoneLiability
Namespace Prefix:	CLSN_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration