Share Capital and Premium Related to the Share Capitals - Schedule of Share Capital Information (Parenthetical) (Detail)	6 Months Ended
Jun. 30, 2026 shares
Disclosure of classes of share capital [abstract]
Shares issued to the benefit of employees related free share plans	358,717

The increase in the number of ordinary shares issued due to vesting of free shares granted to employees and directors in share-based payment arrangements.

No definition available.

Name:	clls_IncreaseInNumberOfOrdinarySharesIssuedDueToVestingOfFreeShares
Namespace Prefix:	clls_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration