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Investments Accounted for Under Equity Method - Summary of Guarantees and Commitments (Detail) - Constellium-Bowling Green [member] - EUR (€) € in Millions	Dec. 31, 2018	Dec. 31, 2017
Financial guarantees [member]
Disclosure of joint ventures [line items]
Total guarantees and commitments given	€ 11	€ 11
Loan Facility Commitment [member]
Disclosure of joint ventures [line items]
Total guarantees and commitments given	3
Supplier guarantees [member]
Disclosure of joint ventures [line items]
Total guarantees and commitments given	€ 3	€ 3

X

- Definition

The commitments that the entity has relating to its joint ventures as specified in paragraphs B18-B20 of IFRS 12. [Refer: Joint ventures [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 12
-IssueDate 2018-01-01
-Paragraph 23
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=12&date=2018-03-01&anchor=para_23_a&doctype=Standard
-URIDate 2018-03-16

+ Details

Name:	ifrs-full_CommitmentsInRelationToJointVentures
Namespace Prefix:	ifrs-full_
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_DisclosureOfJointVenturesLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	ifrs-full_JointVenturesAxis=cstm_ConstelliumUACJABSLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=cstm_FinancialGuaranteesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=cstm_LoanFacilityCommitmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=cstm_SupplierGuaranteesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: