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Earn-out Milestone Liability - Schedule of Risk Adjustment Assessment (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
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%
Minimum [Member]
Estimated Time to Achieve	3 months 15 days	1 year 15 days	6 months 14 days	1 year 1 month 13 days
Maximum [Member]
Estimated Time to Achieve	1 year 3 months 15 days	2 years 15 days	1 year 6 months 14 days	2 years 1 month 13 days

X

- Definition

Discount Rate on milestone liabilities.

+ References

No definition available.

+ Details

Name:	CLSN_DiscountRatesOnMileStoneLiabilities
Namespace Prefix:	CLSN_
Data Type:	dtr-types: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_RiskadjustmentAssessmentOfAchievingMilestone
Namespace Prefix:	CLSN_
Data Type:	dtr-types: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: