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v3.10.0.1

Borrowings - Additional Information (Detail) | 6 Months Ended
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Jun. 30, 2018
Pan US ABL facility (due 2022) [member] |
Disclosure of detailed information about borrowings [line items] |
Debt covenant, Percentage of aggregate revolving loan commitments | 10.00%

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- Definition Percentage of excess availability vs. aggregate revolving commitment that require an evaluation of compliance. + References No definition available. + Details
Name: | cstm_PercentageOfRevolvingLoanCommitments
Namespace Prefix: | cstm_
Data Type: | num:percentItemType
Balance Type: | na
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

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