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COVID-19-RELATED GOVERNMENT ASSISTANCE (Details) € in Millions | 6 Months Ended
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Jun. 30, 2020 EUR (€)
Government Assistance [Abstract] |
Proceeds From Government Assistance | € 14

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- Definition Government Assistance [Abstract] + References No definition available. + Details
Name: | cstm_GovernmentAssistanceAbstract
Namespace Prefix: | cstm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Proceeds From Government Assistance, COVID-19 + References No definition available. + Details
Name: | cstm_ProceedsFromGovernmentAssistanceCOVID19
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration