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v3.21.1

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

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- Definition Discount Rate on milestone liabilities. + References No definition available. + Details
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Namespace Prefix: | CLSN_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Estimated time to achieve milestone liability. + References No definition available. + Details
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Namespace Prefix: | CLSN_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- 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

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

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