XML 133 R91.htm IDEA: XBRL DOCUMENT
v3.19.1

Other Gains / (Losses)-Net - Summary of Other Gains/(Losses) Net (Parenthetical) (Detail) - EUR (€) € in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Jul. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Analysis of other gains losses [line items] |  |  |  |
Gains / (Losses) on pension plans amendments |  | € 36 | € 20 |
Cash payment received on acquisition |  |  |  | € 21
Gain net of costs on acquisition |  |  |  | € 20
Disposal of leased assets [member] | Novelis [member] |  |  |  |
Analysis of other gains losses [line items] |  |  |  |
Sale of assets | € 200 |  |  |
Net gain on sale of assets | € 190 |  |  |

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

X
- Definition Analysis of other gains losses [line items] + References No definition available. + Details
Name: | cstm_AnalysisOfOtherGainsLossesLineItems
Namespace Prefix: | cstm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Cash payment received upon acquisition. + References No definition available. + Details
Name: | cstm_CashPaymentReceivedUponAcquisition
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Gains (losses) on pension plan amendments. + References No definition available. + Details
Name: | cstm_GainsLossesOnPensionPlanAmendments
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The gains (losses) on disposals of property, plant and equipment. [Refer: Property, plant and equipment] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2018-01-01 -Paragraph 98 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_98_c&doctype=Standard -URIDate 2018-03-16 + Details
Name: | ifrs-full_GainsLossesOnDisposalsOfPropertyPlantAndEquipment
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The cash inflow from the disposal of property, plant and equipment, intangible assets other than goodwill, investment property and other non-current assets. [Refer: Intangible assets other than goodwill; Investment property; Other non-current assets; Property, plant and equipment] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 7 -IssueDate 2018-01-01 -Paragraph 16 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=7&date=2018-03-01&anchor=para_16&doctype=Standard -URIDate 2018-03-16 + Details
Name: | ifrs-full_ProceedsFromDisposalsOfPropertyPlantAndEquipmentIntangibleAssetsOtherThanGoodwillInvestmentPropertyAndOtherNoncurrentAssets
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | ifrs-full_ArrangementsInvolvingLegalFormOfLeaseAxis=cstm_DisposalOfLeasedAssetsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ifrs-full_SignificantInvestmentsInSubsidiariesAxis=cstm_NovelisMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |