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Non Cash Investing & Financing Transactions - Additional Information (Detail) - EUR (€) € in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Disclosure Of Non Cash Investing And Financing Transactions [abstract] |  |  |
Property, plant and equipment acquired by finance lease | € 28 | € 17 | € 16
Fair value of PSU and RSUs vested | € 8 | € 1 | € 4

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- Definition Additions through noncash finance lease, property, plant and equipment. + References No definition available. + Details
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Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Disclosure Of Non Cash Investing And Financing Transactions + References No definition available. + Details
Name: | cstm_DisclosureOfNonCashInvestingAndFinancingTransactionsAbstract
Namespace Prefix: | cstm_
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- Definition Fair value of performance stock units and restricted stock units vested. + References No definition available. + Details
Name: | cstm_FairValueOfPerformanceStockUnitsAndRestrictedStockUnitsVested
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration