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Borrowings - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2019
Pan US ABL facility (due 2022) [member]
Disclosure of detailed information about borrowings [line items]
Debt covenant, Percentage of aggregate revolving loan commitments	10.00%

X

- Definition

Percentage of excess availability vs. aggregate revolving commitment that require an evaluation of compliance.

+ References

No definition available.

+ Details

Name:	cstm_PercentageOfRevolvingLoanCommitments
Namespace Prefix:	cstm_
Data Type:	num:percentItemType
Balance Type:	na
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_PanUsAblFacilityDueTwoThousandTwentyTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: