Represents the reduction of the contractual obligation that will occur nineteen months from initial term.

No definition available.

Name:	CLSN_ContractualObligationReductionNineteenMonthsFromInitialTerm
Namespace Prefix:	CLSN_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The remaining amount due under contractual obligations.

+ References

No definition available.

+ Details

Name:	CLSN_ContractualObligationRemaining
Namespace Prefix:	CLSN_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The number of months in which free rent is provided.

+ References

No definition available.

+ Details

Name:	CLSN_NumberOfMonthsInWhichFreeRentIsProvided
Namespace Prefix:	CLSN_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the payment to be paid each month under an operating lease.

+ References

No definition available.

+ Details

Name:	CLSN_OperatingLeasesMonthlyRentPayment
Namespace Prefix:	CLSN_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the payment to be paid each month in the final year under an operating lease.

+ References

No definition available.

+ Details

Name:	CLSN_OperatingLeasesMonthlyRentPaymentInFinalYear
Namespace Prefix:	CLSN_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the payment to be paid each month in the first year under an operating lease.

+ References

No definition available.

+ Details

Name:	CLSN_OperatingLeasesMonthlyRentPaymentInFirstYear
Namespace Prefix:	CLSN_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments.

+ References

No definition available.

+ Details

Name:	us-gaap_ContractualObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeOperatingLeaseTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Cash payments to lessor's for use of assets under operating leases.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3536-108585

+ Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_StandbyLettersOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LeaseArrangementTypeAxis=CLSN_LawrencevilleNewJerseyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LeaseArrangementTypeAxis=CLSN_HuntsvilleAlabamaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=CLSN_EGENAssetPurchaseAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Contingent Liabilities and Commitments (Details Narrative)	1 Months Ended
	Jan. 31, 2018 USD ($) ft²	Apr. 30, 2017 USD ($) ft²	Apr. 30, 2012 USD ($)	Jul. 31, 2011 ft²	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2014 ft²
Irrevocable and unconditional standby letter of credit					$ 908,556	$ 665,373
Standby Letter of Credit [Member]
Contractual obligation reduction nineteen months from initial term			$ 50,000
Contractual obligation remaining			100,000
Lawrenceville New Jersey [Member]
Area of premises \| ft²		7,565		10,870
Lease term		64 months		66 months
The number of months in which free rent is provided		4 months		6 months
Payment for rent			23,000
Operating leases, monthly rent payment in first year		$ 18,900
Operating leases, monthly rent payment in final year		$ 20,500
Lawrenceville New Jersey [Member] \| Standby Letter of Credit [Member]
Irrevocable and unconditional standby letter of credit			$ 250,000
Huntsville Alabama [Member] \| EGEN Asset Purchase Agreement [Member]
Area of premises \| ft²	9,049						11,500
Lease term	60 months
Operating leases, monthly rent payment	$ 18,100