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Trade Payables and Other - Additional Information (Detail) € in Millions	6 Months Ended
	Jun. 30, 2019 EUR (€)
Receivables from contracts with customers [abstract]
Revenue recognized related to contract liabilities	€ 25
Deferred revenue related to contract liabilities	€ 30

X

- Definition

Deferred revenue related to contract liabilities.

+ References

No definition available.

+ Details

Name:	cstm_DeferredRevenueRelatedToContractLiabilities
Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Revenue recognized related to contract liabilities.

+ References

No definition available.

+ Details

Name:	cstm_RevenueRecognizedRelatedToContractLiabilities
Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- References

No definition available.

+ Details

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