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Other Gains / (Losses) - Net - Summary of Other Gains/(Losses) Net (Parenthetical) (Detail) € in Millions	12 Months Ended
	Dec. 31, 2016 EUR (€)
Analysis of income and expense [abstract]
Cash payment received on acquisition	€ 21
Gain net of costs on acquisition	€ 20

X

- Definition

Acquisition purchase price adjustment.

+ References

No definition available.

+ Details

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

X

- Definition

Cash payment received upon acquisition.

+ References

No definition available.

+ Details

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

X

- References

No definition available.

+ Details

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