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Parent Company - Additional Information (Detail) - IFRS9 [member] - EUR (€) € in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Disclosure of condensed financial information of parent company [line items]
Loss on opening retained earnings, net of tax	€ 151
Loss allowance		€ 0
Gain on other comprehensive income	€ 53

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- Definition

Line item represents disclosure of condensed financial information of parent company.

+ References

No definition available.

+ Details

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Namespace Prefix:	cstm_
Data Type:	xbrli:stringItemType
Balance Type:	na
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- Definition

Expected credit loss allowance.

+ References

No definition available.

+ Details

Name:	cstm_ExpectedCreditLossAllowance
Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- Definition

Gain (loss) on opening retained earnings net.

+ References

No definition available.

+ Details

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

X

- Definition

Gain (loss) on other comprehensive income.

+ References

No definition available.

+ Details

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

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- Details

Name:	ifrs-full_InitiallyAppliedIFRSsAxis=cstm_IFRS9Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: