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TRADE PAYABLES AND OTHER - Additional Information (Details) € in Millions | 6 Months Ended
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Jun. 30, 2020 EUR (€)
Trade and other payables [abstract] |
Revenue recognized related to contract liabilities | € 14
Deferred revenue related to contract liabilities | € 25

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- 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

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- Definition The amount of revenue that was included in the contract liability balance at the beginning of the period. [Refer: Contract liabilities; Revenue from contracts with customers] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 15 -IssueDate 2019-01-01 -Paragraph 116 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=15&code=ifrs-tx-2019-en-r&anchor=para_116_b&doctype=Standard -URIDate 2019-03-27 + Details
Name: | ifrs-full_RevenueThatWasIncludedInContractLiabilityBalanceAtBeginningOfPeriod
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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