XML 212 R170.htm IDEA: XBRL DOCUMENT
v3.19.1

Share Capital - Schedule of Share Capital (Parenthetical) (Detail) | 12 Months Ended
--- | ---
Dec. 31, 2018 shares
Disclosure of classes of share capital [line items] |
Share issued | 1,488,771
Common shares [member] | Employees [member] |
Disclosure of classes of share capital [line items] |
Share issued | 1,488,771
Ordinary shares [member] |
Disclosure of classes of share capital [line items] |
Treasury shares held | 38,597

X
- Definition Number of auto-controlled shares. + References No definition available. + Details
Name: | cstm_NumberOfSharesIssuedAndGranted
Namespace Prefix: | cstm_
Data Type: | xbrli:sharesItemType
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_DisclosureOfClassesOfShareCapitalLineItems
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The increase (decrease) in the number of ordinary shares issued. [Refer: Ordinary shares [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2018-01-01 -Paragraph 112 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_112_c&doctype=Standard -URIDate 2018-03-16 + Details
Name: | ifrs-full_IncreaseDecreaseInNumberOfOrdinarySharesIssued
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | ifrs-full_ClassesOfShareCapitalAxis=cstm_CommonSharesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ifrs-full_DefinedBenefitPlansAxis=cstm_EmployeesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ifrs-full_ClassesOfShareCapitalAxis=ifrs-full_OrdinarySharesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |