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Other Gains / (Losses) - Net - Summary of Other Gains/(Losses) Net (Parenthetical) (Detail) € in Millions | 12 Months Ended
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Dec. 31, 2016 EUR (€)
Analysis of income and expense [abstract] |
Cash payment received on acquisition | € 21
Gain net of costs on acquisition | € 20

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- Definition Acquisition purchase price adjustment. + References No definition available. + Details
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Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Cash payment received upon acquisition. + References No definition available. + Details
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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_AnalysisOfIncomeAndExpenseAbstract
Namespace Prefix: | ifrs-full_
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Balance Type: | na
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