Number of warrants issued.

No definition available.

Name:	clsn_NumberOfWarrantsIssued
Namespace Prefix:	clsn_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of new stock issued with the exercise of some warrants during the period.

+ References

No definition available.

+ Details

Name:	clsn_StockIssuedDuringPeriodSharesExerciseOfWarrants
Namespace Prefix:	clsn_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of securities into which the class of warrant or right may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i)(2))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of warrants or rights outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_ClassOfWarrantOrRightOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The cash inflow associated with the amount received from holders exercising their stock options. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (j)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3255-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Financing Activities
-URI http://asc.fasb.org/extlink&oid=6513228

+ Details

Name:	us-gaap_ProceedsFromStockOptionsExercised
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash inflow associated with the amount received from holders exercising their stock warrants.

+ References

No definition available.

+ Details

Name:	us-gaap_ProceedsFromWarrantExercises
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_SubsidiarySaleOfStockAxis=clsn_June2016CommonStockOfferingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ClassOfWarrantOrRightAxis=clsn_HerculesWarrantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ClassOfWarrantOrRightAxis=clsn_SeriesBWarrantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_CommonStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ClassOfWarrantOrRightAxis=clsn_SeriesAWarrantMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Note 13 - Warrants (Details Textual) - USD ($)	3 Months Ended	12 Months Ended		24 Months Ended
Note 13 - Warrants (Details Textual) - USD ($)	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Jun. 13, 2016	Dec. 31, 2014	Nov. 30, 2013
Number Of Warrants Issued		16,066,371	1,950,000
Class of Warrant or Right, Outstanding		20,831,883	5,894,675	20,831,883		5,069,815
Proceeds from Stock Options Exercised				$ 0
Proceeds from Warrant Exercises		$ 2,500		$ 0
June 2016 Common Stock Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights					8,823,528
Hercules Warrant [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights							194,986
Series B Warrant [Member] \| June 2016 Common Stock Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights					2,100,000
Class of Warrant or Right, Outstanding		1,850,000		1,850,000
Series B Warrant [Member] \| June 2016 Common Stock Offering [Member] \| Common Stock [Member]
Stock Issued During Period, Shares, Exercise of Warrants	250,000
Series A Warrant [Member] \| June 2016 Common Stock Offering [Member]
Class of Warrant or Right, Outstanding					1,850,000