XML 202 R160.htm IDEA: XBRL DOCUMENT
v3.19.1

Pensions and Other Post-employment Benefit Obligations - Details of Net Defined Benefit Obligations by Country (Detail) - EUR (€) € in Millions | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Defined benefit obligations | € 990 | € 1,051 |
Plan assets | (380) | (387) |
Net defined benefit liability | 610 | 664 | € 735
France [member] |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Defined benefit obligations | 151 | 148 |
Plan assets | (3) | (3) |
Net defined benefit liability | 148 | 145 |
Germany [member] |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Defined benefit obligations | 136 | 142 |
Plan assets | (1) | (1) |
Net defined benefit liability | 135 | 141 |
Switzerland [member] |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Defined benefit obligations | 251 | 251 |
Plan assets | (178) | (177) |
Net defined benefit liability | 73 | 74 |
United States [member] |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Defined benefit obligations | 451 | 509 |
Plan assets | (198) | (206) |
Net defined benefit liability | 253 | 303 |
Other countries [member] |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Defined benefit obligations | 1 | 1 |
Net defined benefit liability | € 1 | € 1 |

X
- Definition Defined benefit obligation. + References No definition available. + Details
Name: | cstm_DefinedBenefitObligation
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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_DisclosureOfNetDefinedBenefitLiabilityAssetLineItems
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2018-01-01 -Paragraph 140 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_140_a&doctype=Standard -URIDate 2018-03-16 + Details
Name: | ifrs-full_LiabilityAssetOfDefinedBenefitPlans
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The fair value of defined benefit plan assets. Plan assets comprise assets held by a long-term employee benefit fund and qualifying insurance policies. [Refer: At fair value [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2018-01-01 -Paragraph 57 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_57_a&doctype=Standard -URIDate 2018-03-16 + Details
Name: | ifrs-full_PlanAssetsAtFairValue
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | cstm_CountryNameAxis=country_FR
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cstm_CountryNameAxis=country_DE
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cstm_CountryNameAxis=country_CH
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cstm_CountryNameAxis=country_US
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cstm_CountryNameAxis=cstm_OtherCountriesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |