Note 6 - Fair Value of Measurements (Details)	6 Months Ended
Jun. 30, 2013
Fair Value Disclosures [Abstract]
Fair Value, Assets and Liabilities, Transfer Between Level 1 and Level 2, Level 3 Transfer In or Out, Description	There were no transfers of assets or liabilities between Level 1 and Level 2 and no transfers in or out of Level 3 during the six month period ended June 30, 2013.

Description of the transfers of assets and liabilities measured on a recurring basis between Level 1 and Level 2, and of the transfers out of and into Level 3 of the fair value hierarchy.

No definition available.

Name:	clsn_FairValueAssetsAndLiabilitiesTransferBetweenLevel1AndLevel2Level3TransferInOrOutDescription
Namespace Prefix:	clsn_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration