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PROVISIONS - Narrative (Details)	Sep. 30, 2025
Commitments and Contingencies Disclosure [Abstract]
Provisions, expected settlement, period	40 years

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- Definition

Provisions, Expected Settlement, Period

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No definition available.

+ Details

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Namespace Prefix:	cstm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

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- References

No definition available.

+ Details

Name:	us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration