The amount of estimated time to achieve the milestone.

No definition available.

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

- Definition

The amount of gain (loss) from the change in the fair value of earn-out milestone liability.

+ References

No definition available.

+ Details

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

- Definition

The percentage of risk-adjusted assessment of each milestone.

+ References

No definition available.

+ Details

Name:	clsn_RiskAdjustedAssessmentOfEachMilestone
Namespace Prefix:	clsn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of liabilities incurred by the acquirer as part of consideration transferred in a business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 25
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6405-128476

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6578-128477

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6613-128477

+ Details

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

- Definition

For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

+ Details

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

- Details

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

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Note 13 - Earn-out Milestone Liability (Details Textual) - USD ($)		1 Months Ended	3 Months Ended		12 Months Ended
	Jun. 20, 2014	Jun. 20, 2014	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
EGEN Inc [Member] \| Minimum [Member]
Risk Adjusted Assessment of Each Milestone	10.00%	10.00%
Estimated Time to Achieve the Milestone		1 year 182 days
EGEN Inc [Member] \| Maximum [Member]
Risk Adjusted Assessment of Each Milestone	67.00%	67.00%
Estimated Time to Achieve the Milestone		2 years 182 days
EGEN Inc [Member]
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	$ 30,400,000	$ 30,400,000	$ 30,400,000
Business Combination, Consideration Transferred, Liabilities Incurred	$ 13,900,000	$ 13,878,000	$ 14,200,000	$ 13,800,000	$ 13,900,000	$ 13,700,000
Minimum [Member]
Risk Adjusted Assessment of Each Milestone			10.00%	10.00%	10.00%	10.00%
Estimated Time to Achieve the Milestone			91 days	1 year 62 days	182 days	1 year 91 days
Maximum [Member]
Risk Adjusted Assessment of Each Milestone			75.00%	67.00%	75.00%	75.00%
Estimated Time to Achieve the Milestone			2 years 91 days	6 years 94 days	2 years 182 days	6 years 182 days
Business Combination, Consideration Transferred, Liabilities Incurred			$ 302,656	$ 13,800,000		$ 13,700,000
Gain (Loss) from Change in Fair Value of Earn out Milestone Liability				$ 172,136