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Share-Based Compensation - Summary of Movements in Shares (Parenthetical) (Detail) | 6 Months Ended
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Jun. 30, 2018 shares
Disclosure of movement of potential shares [line items] |
Performance based shares forfeited upon departure of certain beneficiaries | 106,163
Performance based shares forfeited upon non-fulfilment of performance conditions | 28,000
Performance share units (PSUs) [member] |
Disclosure of movement of potential shares [line items] |
Percentage of vesting multiplier | 100.00%

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- Definition Disclosure of movement of potential shares. + References No definition available. + Details
Name: | cstm_DisclosureOfMovementOfPotentialSharesLineItems
Namespace Prefix: | cstm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of other equity instruments forfeited on departure of certain beneficiaries in share based payment arrangement. + References No definition available. + Details
Name: | cstm_NumberOfOtherEquityInstrumentsForfeitedOnDepartureOfCertainBeneficiariesInSharebasedPaymentArrangement
Namespace Prefix: | cstm_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of other equity instruments forfeited on non fulfillment of performance conditions in share based payment arrangement. + References No definition available. + Details
Name: | cstm_NumberOfOtherEquityInstrumentsForfeitedOnNonFulfillmentOfPerformanceConditionsInSharebasedPaymentArrangement
Namespace Prefix: | cstm_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of sharebased payments vesting multiplier. + References No definition available. + Details
Name: | cstm_PercentageOfSharebasedPaymentsVestingMultiplier
Namespace Prefix: | cstm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=cstm_PerformanceShareUnitsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |