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PROPERTY, PLANT AND EQUIPMENT - Narrative (Details) - EUR (€) € in Millions | 6 Months Ended
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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
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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