XML 184 R142.htm IDEA: XBRL DOCUMENT
v3.19.1

Financial Risk Management - Summary of Nominal Value of Currency Derivatives (Detail) - Currency risk [member] EUR_ in Millions | Dec. 31, 2018 EUR_
--- | ---
Forward derivatives sales - USD/EUR [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Maturity period | 2019-2024
Forward derivatives sales - EUR/CHF [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Maturity period | 2019-2023
Forward derivatives sales - Other currencies [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Maturity period | 2019-2020
Forward derivatives purchases - USD/EUR [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Maturity period | 2019-2024
Forward derivatives purchases - EUR/CHF [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Maturity period | 2019-2024
Forward derivatives purchases - EUR/CZK [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Maturity period | 2019-2020
Forward derivatives purchases - Other currencies [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Maturity period | 2019
Less than 1 year [member] | Forward derivatives sales - USD/EUR [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal value | 519
Less than 1 year [member] | Forward derivatives sales - EUR/CHF [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal value | 64
Less than 1 year [member] | Forward derivatives sales - Other currencies [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal value | 17
Less than 1 year [member] | Forward derivatives purchases - USD/EUR [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal value | 543
Less than 1 year [member] | Forward derivatives purchases - EUR/CHF [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal value | 118
Less than 1 year [member] | Forward derivatives purchases - EUR/CZK [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal value | 76
Less than 1 year [member] | Forward derivatives purchases - Other currencies [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal value | 4
Over 1 year [member] | Forward derivatives sales - USD/EUR [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal value | 276
Over 1 year [member] | Forward derivatives sales - EUR/CHF [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal value | 14
Over 1 year [member] | Forward derivatives sales - Other currencies [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal value | 1
Over 1 year [member] | Forward derivatives purchases - USD/EUR [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal value | 93
Over 1 year [member] | Forward derivatives purchases - EUR/CHF [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal value | 62
Over 1 year [member] | Forward derivatives purchases - EUR/CZK [member] |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |
Nominal value | 61

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 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
- Definition The nominal amount of a hedging instrument. [Refer: Hedging instruments [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2018-01-01 -Paragraph 23B -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2018-03-01&anchor=para_23B_a&doctype=Standard -URIDate 2018-03-16 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2018-01-01 -Paragraph 24A -Subparagraph d -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2018-03-01&anchor=para_24A_d&doctype=Standard -URIDate 2018-03-16 + Details
Name: | ifrs-full_NominalAmountOfHedgingInstrument
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

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

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

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

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

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

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

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

X
- Details
Name: | ifrs-full_HedgingInstrumentsAxis=cstm_OtherCurrenciesForwardDerivativesPurchasesMember
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: |