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v3.19.1

Trade Payables and Other - Additional Information (Detail) € in Millions | 12 Months Ended
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Dec. 31, 2018 EUR (€)
Receivables from contracts with customers [abstract] |
Revenue recognized related to contract liabilities | € 55
Deferred revenue related to contract liabilities | € 45

X
- Definition Deferred revenue related to contract liabilities. + References No definition available. + Details
Name: | cstm_DeferredRevenueRelatedToContractLiabilities
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Revenue recognized related to contract liabilities. + References No definition available. + Details
Name: | cstm_RevenueRecognizedRelatedToContractLiabilities
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
Name: | ifrs-full_ReceivablesFromContractsWithCustomersAbstract
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration