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PROPERTY, PLANT AND EQUIPMENT - Narrative (Details) - EUR (€) € in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Property, plant and equipment [abstract]
Expense related to short-term leases, low value asset leases and variable lease still recognized as operating expenses	€ 5	€ 6

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- Definition

Expense Relating To Short-term Leases, Leases Of Low-value Assets And Variable Lease Payments For Which Recognition Exemption Has Been Used

+ References

No definition available.

+ Details

Name:	cstm_ExpenseRelatingToShorttermLeasesLeasesOfLowvalueAssetsAndVariableLeasePaymentsForWhichRecognitionExemptionHasBeenUsed
Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- References

No definition available.

+ Details

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