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Operating Segment Information - Summary of Segment Revenue (Parenthetical) (Detail) € in Millions	12 Months Ended
	Dec. 31, 2016 EUR (€)
Holdings & Corporate [member]
Disclosure of operating segments [line items]
One-time payment related to the re-negotiation of contract with customer	€ 20

X

- Definition

One time payment related to renegotiation of contract with customer.

+ References

No definition available.

+ Details

Name:	cstm_OneTimePaymentRelatedToRenegotiationOfContractWithCustomer
Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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_DisclosureOfOperatingSegmentsLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	ifrs-full_SegmentsAxis=cstm_HoldingsAndCorporateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: