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Earnings Per Share - Summary of Number of Shares Attributable to Equity Holders (Parenthetical) (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Weighted average ordinary shares and adjusted weighted average ordinary shares [abstract]
Potential Ordinary shares	3,291,875	411,902

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- Definition

Calculation of diluted potential ordinary shares.

+ References

No definition available.

+ Details

Name:	cstm_CalculationOfDilutedPotentialOrdinaryShares
Namespace Prefix:	cstm_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

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- References

No definition available.

+ Details

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