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Borrowings - Summary of Borrowings (Parenthetical) (Detail) € in Millions	Mar. 28, 2018 EUR (€)
BPI unsecured revolving credit facilities [member]
Disclosure of detailed information about borrowings [line items]
Borrowing capacity	€ 10

X

- Definition

Borrowing capacity under revolving credit facility.

+ References

No definition available.

+ Details

Name:	cstm_BorrowingCapacityUnderRevolvingCreditFacility
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_DisclosureOfDetailedInformationAboutBorrowingsLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	ifrs-full_BorrowingsByNameAxis=cstm_BPIUnsecuredRevolvingCreditFacilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: