XML 110 R103.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Pensions and Other Post-employment Benefit Obligations - Details of Net Defined Benefit Obligations by Country (Detail) - EUR (€) € in Millions | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Disclosure of net defined benefit liability (asset) [line items] |  |
Defined benefit obligations | € 1,030 | € 1,051
Plan assets | (387) | (387)
Net defined benefit liability | 643 | 664
France [member] |  |
Disclosure of net defined benefit liability (asset) [line items] |  |
Defined benefit obligations | 149 | 148
Plan assets | (3) | (3)
Net defined benefit liability | 146 | 145
Germany [member] |  |
Disclosure of net defined benefit liability (asset) [line items] |  |
Defined benefit obligations | 139 | 142
Plan assets | (1) | (1)
Net defined benefit liability | 138 | 141
Switzerland [Member] |  |
Disclosure of net defined benefit liability (asset) [line items] |  |
Defined benefit obligations | 250 | 251
Plan assets | (179) | (177)
Net defined benefit liability | 71 | 74
United States [member] |  |
Disclosure of net defined benefit liability (asset) [line items] |  |
Defined benefit obligations | 491 | 509
Plan assets | (204) | (206)
Net defined benefit liability | 287 | 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 2017-01-01 -Paragraph 140 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_140_a&doctype=Standard -URIDate 2017-03-09 + 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 2017-01-01 -Paragraph 57 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_57_a&doctype=Standard -URIDate 2017-03-09 + 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=currency_CHE
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: |