Represents period after the date of issuance when warrants become exercisable.

No definition available.

Name:	clsn_ClassOfWarrantOrRightExercisableTerm
Namespace Prefix:	clsn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of warrants that institutional investors have agreed to waive their rights to exercise.

+ References

No definition available.

+ Details

Name:	clsn_ClassOfWarrantOrRightWaived
Namespace Prefix:	clsn_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of shares of common stock and warrants issued during period.

+ References

No definition available.

+ Details

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

- Definition

Carrying value as of the balance sheet date of common stock warrant liability.

+ References

No definition available.

+ Details

Name:	clsn_CommonStockWarrantLiability
Namespace Prefix:	clsn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Common Stock Warrant Liability Benefit from Adjustment in Warrant Liability.

+ References

No definition available.

+ Details

Name:	clsn_CommonStockWarrantLiabilityBenefitfromAdjustmentInWarrantLiability
Namespace Prefix:	clsn_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of warrant issued.

+ References

No definition available.

+ Details

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

- Definition

Represents net proceeds from the issuance of common stock, preferred stock, treasury stock, stock options, and other types of equity.

+ References

No definition available.

+ Details

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Namespace Prefix:	clsn_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

Reclassification of financial instrument from equity to liability.

+ References

No definition available.

+ Details

Name:	clsn_ReclassificationFromEquitytoLiability
Namespace Prefix:	clsn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

In connection with the reverse stock split, warrants reclassified from liability to equity.

+ References

No definition available.

+ Details

Name:	clsn_ReverseStockSplitReclassificationFromLiabilityToEquity
Namespace Prefix:	clsn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Exercise price per share or per unit of warrants or rights outstanding.

+ References

+ Details

Name:	us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- 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

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph i

-Subparagraph 2

-Article 4

+ Details

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

- Definition

The number of new shares issued in the conversion of stock in a noncash (or part noncash) transaction. Noncash is defined as transactions during a period that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash inflow from the issuance of common stock, preferred stock, treasury stock, stock options, and other types of equity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

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=31042434&loc=d3e3255-108585

+ Details

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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_ClassOfWarrantOrRightAxis=clsn_SeptemberTwoThousandNineWarrantsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=clsn_HerculesCreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=clsn_OxfordAndHorizonCreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=clsn_WaivedWarrantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=clsn_WarrantLiabilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsidiarySaleOfStockAxis=clsn_SeptemberTwoThousandNineRegisteredDirectOfferingMember
Namespace Prefix:
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Period Type:

Note 13 - Warrants (Details) (USD $)	0 Months Ended	1 Months Ended	12 Months Ended			1 Months Ended
Note 13 - Warrants (Details) (USD $)	Jun. 03, 2013	Oct. 28, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2009	Dec. 31, 2011	May 30, 2013	Nov. 30, 2013	Jun. 30, 2012
Note 13 - Warrants (Details) [Line Items]
Number of Warrants Issued (in Shares)			1,801,802clsn_NumberOfWarrantsIssued	1,341,382clsn_NumberOfWarrantsIssued
Proceeds from Warrant Exercises			$ 0us-gaap_ProceedsFromWarrantExercises	$ 261,944us-gaap_ProceedsFromWarrantExercises	$ 10,106,557us-gaap_ProceedsFromWarrantExercises
Conversion of Stock, Shares Issued (in Shares)				15,833us-gaap_ConversionOfStockSharesIssued1	847,715us-gaap_ConversionOfStockSharesIssued1
Class of Warrant or Right, Exercise Price of Warrants or Rights (in Dollars per share)			$ 8.18	$ 10.65	$ 15.17		$ 1,418
Reclassification From Equity to Liability	9,100,000	5,300,000	476,261	9,110,302
undefined				5,300,572
Common Stock Warrant Liability			275,008clsn_CommonStockWarrantLiability	3,026clsn_CommonStockWarrantLiability	4,283,932clsn_CommonStockWarrantLiability		166,398clsn_CommonStockWarrantLiability
September 2009 Warrants [Member] \| Maximum [Member]
Note 13 - Warrants (Details) [Line Items]
Class of Warrant or Right, Exercisable Term						66 months
September 2009 Warrants [Member] \| Minimum [Member]
Note 13 - Warrants (Details) [Line Items]
Class of Warrant or Right, Exercisable Term						6 months
September 2009 Warrants [Member]
Note 13 - Warrants (Details) [Line Items]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in Shares)						224,239
Class of Warrant or Right, Exercise Price of Warrants or Rights (in Dollars per share)						23.58
Waived Warrant [Member]
Note 13 - Warrants (Details) [Line Items]
Class of Warrant or Right, Waived (in Shares)								1,398,816
September 2009 Registered Direct Offering [Member]
Note 13 - Warrants (Details) [Line Items]
Proceeds from Issuance or Sale of Equity						7,100,000
Proceeds from Issuance or Sale of Equity, Net						6,300,000
Common Stock and Warrants Issued, Shares (in Shares)						484,478
Waived Warrant [Member]
Note 13 - Warrants (Details) [Line Items]
undefined		5,300,000
Warrant Liability [Member]
Note 13 - Warrants (Details) [Line Items]
undefined			$ 204,279	$ 8,100,000	$ (4,100,000)
Hercules Credit Agreement [Member]
Note 13 - Warrants (Details) [Line Items]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in Shares)									194,986
Oxford and Horizon Credit Agreement [Member]
Note 13 - Warrants (Details) [Line Items]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in Shares)										11,415