XML 178 R141.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Financial Risk Management - Summary of Nominal Value of Commodity Derivatives (Detail) - Commodity price risk [member] € in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 EUR (€)
Aluminium [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Maturity Period | 2018-2022
Premiums [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Maturity Period | 2018-2021
Copper [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Maturity Period | 2018
Silver [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Maturity Period | 2018
Natural gas [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Maturity Period | 2018
Zinc [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Maturity Period | 2018
Less than 1 year [member] | Aluminium [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal amount of derivatives | € 335
Less than 1 year [member] | Premiums [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal amount of derivatives | 5
Less than 1 year [member] | Copper [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal amount of derivatives | 3
Less than 1 year [member] | Silver [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal amount of derivatives | 7
Less than 1 year [member] | Natural gas [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal amount of derivatives | 4
Less than 1 year [member] | Zinc [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal amount of derivatives | 10
Over 1 year [member] | Aluminium [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal amount of derivatives | 43
Over 1 year [member] | Premiums [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal amount of derivatives | € 7

X
- Definition Derivative instrument maturity period. + References No definition available. + Details
Name: | cstm_DerivativeInstrumentMaturityPeriod
Namespace Prefix: | cstm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The nominal amount of a credit derivative. [Refer: Derivatives [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Note Effective 2018-01-01 -Name IFRS -Number 7 -IssueDate 2017-01-01 -Paragraph 24G -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_24G_a&doctype=Standard -URIDate 2017-03-09 + Details
Name: | ifrs-full_CreditDerivativeNominalAmount
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

X
- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
Name: | ifrs-full_DisclosureOfInformationAboutTermsAndConditionsOfHedgingInstrumentsAndHowTheyAffectFutureCashFlowsLineItems
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | ifrs-full_TypesOfRisksAxis=ifrs-full_CommodityPriceRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ifrs-full_HedgingInstrumentsAxis=cstm_AluminiumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ifrs-full_HedgingInstrumentsAxis=cstm_PremiumsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ifrs-full_HedgingInstrumentsAxis=cstm_CopperMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ifrs-full_HedgingInstrumentsAxis=cstm_SilverMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ifrs-full_HedgingInstrumentsAxis=cstm_NaturalGasMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ifrs-full_HedgingInstrumentsAxis=cstm_ZincMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ifrs-full_MaturityAxis=ifrs-full_NotLaterThanOneYearMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ifrs-full_MaturityAxis=ifrs-full_LaterThanOneYearMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |