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Borrowings - Summary of Borrowings (Parenthetical) (Detail) € in Millions | Mar. 28, 2018 EUR (€)
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BPI unsecured revolving credit facilities [member] |
Disclosure of detailed information about borrowings [line items] |
Borrowing capacity | € 10

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- Definition Borrowing capacity under revolving credit facility. + References No definition available. + Details
Name: | cstm_BorrowingCapacityUnderRevolvingCreditFacility
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
Name: | ifrs-full_DisclosureOfDetailedInformationAboutBorrowingsLineItems
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | ifrs-full_BorrowingsByNameAxis=cstm_BPIUnsecuredRevolvingCreditFacilitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |