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NON-CASH INVESTING AND FINANCING TRANSACTIONS (Details) - EUR (€) € in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2020 | Dec. 31, 2019
Disclosure of cash flow statement [Abstract] |  |
Property, plant and equipment acquired by finance lease | € 25 | € 33
Fair value of vested PSUs and RSUs | € 0 | € 4

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- Definition Additions through noncash finance lease, property, plant and equipment. + References No definition available. + Details
Name: | cstm_AdditionsThroughNoncashFinanceLeasePropertyPlantAndEquipment
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Disclosure of cash flow statement + References No definition available. + Details
Name: | cstm_DisclosureofcashflowstatementAbstract
Namespace Prefix: | cstm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Fair value of performance stock units and restricted stock units vested. + References No definition available. + Details
Name: | cstm_FairValueOfPerformanceStockUnitsAndRestrictedStockUnitsVested
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration