Note 5 - Acquisition of EGEN, Inc. (Details) - Pro Forma Information - EGEN, Inc. [Member] - USD ($)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Note 5 - Acquisition of EGEN, Inc. (Details) - Pro Forma Information [Line Items]
Revenues	$ 500,000	$ 627,628
Loss from operations	(24,898,725)	(18,247,339)
Net loss applicable to common shareholders	$ (25,662,572)	$ (15,952,832)

No definition available.

Name:	clsn_Note5AcquisitionofEGENIncDetailsProFormaInformationLineItems
Namespace Prefix:	clsn_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The pro forma income from continuing operations before changes in accounting and extraordinary items, net of tax effect, for the period as if the business combination or combinations had been completed at the beginning of a period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(2)-(3)
-URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463

+ Details

Name:	us-gaap_BusinessAcquisitionsProFormaIncomeLossFromContinuingOperationsBeforeChangesInAccountingAndExtraordinaryItemsNetOfTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.

+ References

+ Details

Name:	us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.

+ References

+ Details

Name:	us-gaap_BusinessAcquisitionsProFormaRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=clsn_EGENIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: