Note 12 - Earn-out Milestone Liability - Summary of the Changes in the Earn-out Milestone Liability (Details)	9 Months Ended
Sep. 30, 2017 USD ($)
Balance	$ 13,188,226
Non-cash benefit from the adjustment for the change in fair value included in net loss	(1,246,151)
Balance	$ 12,518,054

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