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Share-Based Compensation - Summary of Weighted-Average Fair Value (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2017 shares
Disclosure of movement of potential shares [line items]
Performance based shares forfeited upon departure of certain beneficiaries	175,837
Performance based shares forfeited upon non-fulfilment of performance conditions	78,667
Performance share units (PSUs) [member]
Disclosure of movement of potential shares [line items]
Percentage of vesting multiplier	100.00%

X

- Definition

Disclosure of movement of potential shares.

+ References

No definition available.

+ Details

Name:	cstm_DisclosureOfMovementOfPotentialSharesLineItems
Namespace Prefix:	cstm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Number of other equity instruments forfeited on departure of certain beneficiaries in share based payment arrangement.

+ References

No definition available.

+ Details

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

X

- Definition

Number of other equity instruments forfeited on non fulfillment of performance conditions in share based payment arrangement.

+ References

No definition available.

+ Details

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

X

- Definition

Percentage of sharebased payments vesting multiplier.

+ References

No definition available.

+ Details

Name:	cstm_PercentageOfSharebasedPaymentsVestingMultiplier
Namespace Prefix:	cstm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=cstm_PerformanceShareUnitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: