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Earn-out Milestone Liability - Schedule of Risk Adjustment Assessment (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Risk-adjustment Assessment of achieving each Milestone	80.00%	80.00%	80.00%	80.00%
Discount Rate	9.00%	9.00%	9.00%	9.00%
Estimated Time to Achieve				1 year 2 months 30 days
Minimum [Member]
Estimated Time to Achieve	1 year 15 days	1 year	1 year 1 month 13 days
Maximum [Member]
Estimated Time to Achieve	2 years 15 days	2 years	2 years 1 month 13 days

X

- Definition

Discount rate on milestone liabilites.

+ References

No definition available.

+ Details

Name:	CLSN_DiscountRateOnMileStoneLiabilities
Namespace Prefix:	CLSN_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Estimated time to achieve milestone liability.

+ References

No definition available.

+ Details

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

X

- Definition

Risk-adjustment assessment of achieving each milestone.

+ References

No definition available.

+ Details

Name:	CLSN_RiskadjustmentAssessmentOfAchievingEachMilestone
Namespace Prefix:	CLSN_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: