Aggregate credit facility.

No definition available.

Name:	clls_AggregateCreditFacility
Namespace Prefix:	clls_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Agreements Extended Maturity Date

+ References

No definition available.

+ Details

Name:	clls_AgreementsExtendedMaturityDate
Namespace Prefix:	clls_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of the certain Research Tax Credit Receivables subject to a prefinancing.

+ References

No definition available.

+ Details

Name:	clls_PercentageOfFinancingTaxReceivableRelatedToResearchTaxCredit
Namespace Prefix:	clls_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash outflow for repayments of advances and loans from related parties. [Refer: Total for all related parties [member]]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 7
-IssueDate 2025-01-01
-Paragraph 17
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=7&code=ifrs-tx-2025-en-r&anchor=para_17&doctype=Standard
-URIDate 2025-03-27

+ Details

Name:	ifrs-full_CashRepaymentsOfAdvancesAndLoansFromRelatedParties
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

+ References

No definition available.

+ Details

Name:	ifrs-full_DisclosureOfTransactionsBetweenRelatedPartiesLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of expense arising from interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2025-01-01
-Paragraph 28
-Subparagraph e
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=8&code=ifrs-tx-2025-en-r&anchor=para_28_e&doctype=Standard
-URIDate 2025-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 12
-IssueDate 2025-01-01
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-Subparagraph f
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=12&code=ifrs-tx-2025-en-r&anchor=para_B13_f&doctype=Appendix&subtype=B
-URIDate 2025-03-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2025-01-01
-Paragraph 23
-Subparagraph d
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=8&code=ifrs-tx-2025-en-r&anchor=para_23_d&doctype=Standard
-URIDate 2025-03-27

+ Details

Name:	ifrs-full_InterestExpense
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow for interest paid, classified as financing activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 7
-IssueDate 2025-01-01
-Paragraph 31
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=7&code=ifrs-tx-2025-en-r&anchor=para_31&doctype=Standard
-URIDate 2025-03-27

+ Details

Name:	ifrs-full_InterestPaidClassifiedAsFinancingActivities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash inflow from borrowings obtained. [Refer: Borrowings]

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 7
-IssueDate 2025-01-01
-Paragraph 17
-Subparagraph c
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=7&code=ifrs-tx-2025-en-r&anchor=para_17_c&doctype=Standard
-URIDate 2025-03-27

+ Details

Name:	ifrs-full_ProceedsFromBorrowingsClassifiedAsFinancingActivities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	ifrs-full_BorrowingsByNameAxis=clls_StateGuaranteedLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_BorrowingsByNameAxis=clls_BPIRefundableAdvanceAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	clls_MajorShareholdersAxis=clls_BpifranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_BorrowingsByNameAxis=clls_ResearchTaxCreditFinancingsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Related parties and other major shareholders - Additional Information (Details) $ in Thousands, € in Millions						1 Months Ended	6 Months Ended			12 Months Ended
	Dec. 06, 2024 USD ($)	Oct. 06, 2023 USD ($)	Jun. 19, 2023 USD ($)	Jul. 31, 2020 USD ($)	Jul. 31, 2020 EUR (€)	Aug. 31, 2023 EUR (€)	Jun. 30, 2026 USD ($)	Jun. 30, 2026 EUR (€)	Jun. 30, 2025 USD ($)	Dec. 31, 2022
Disclosure of transactions between related parties [line items]
Interest expense							$ 176		$ 344
State Guaranteed Loan [Member]
Disclosure of transactions between related parties [line items]
Increase in borrowings				$ 21,100	€ 18.5
BPI Refundable Advance Agreement [Member]
Disclosure of transactions between related parties [line items]
Increase in borrowings	$ 2,100	$ 1,900	$ 900
Bpifrance [Member]
Disclosure of transactions between related parties [line items]
Financing tax receivable percentage										80.00%
Bpifrance [Member] \| Research Tax Credit Financings [Member]
Disclosure of transactions between related parties [line items]
Interest expense							$ 100
Increase in borrowings \| €						€ 5.3
Agreements extended maturity date							Oct. 15, 2026	Oct. 15, 2026
Bpifrance [Member] \| State Guaranteed Loan [Member]
Disclosure of transactions between related parties [line items]
Loan principal payment							$ 900	€ 0.8
Bpifrance [Member] \| BPI Refundable Advance Agreement [Member]
Disclosure of transactions between related parties [line items]
Loan principal payment							0
Interest expense							$ 200	0.2
Aggregate credit facility \| €								€ 6.4