Borrowings - Summary of Reconciliation of Movement in Net Borrowings Explanatory (Detail) - EUR (€) € in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Disclosure of detailed information about borrowings [abstract]
Beginning Balance	€ 2,127	€ 2,468	€ 2,468
Proceeds from issuance of Senior Notes		610	1,440
Repayments of Senior Notes		€ (610)	(1,559)
Proceeds / (Repayments) from U.S. Revolving Credit Facilities and other loans	10		29
Arrangement fees payment			(29)
Finance lease repayment and others	(7)		(13)
Movement in interests accrued or capitalized	11		(13)
New finance leases	1		17
Deferred arrangement fees and step-up amortization	2		7
Effects of changes in foreign exchange rates	40		(220)
Ending Balance	€ 2,184		€ 2,127

Arrangement fees payment.

No definition available.

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

- Definition

The effect of changes in foreign exchange rates on borrowings.

+ References

No definition available.

+ Details

Name:	cstm_BorrowingsEffectOfChangesInForeignExchangeRates
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Deferred arrangement fees and step-up amortization.

+ References

No definition available.

+ Details

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

- Definition

Finance leases.

+ References

No definition available.

+ Details

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

- Definition

Movement in accrued interest and interest cost capitalized.

+ References

No definition available.

+ Details

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

- Definition

Repayment of obligations under finance leases and other debts.

+ References

No definition available.

+ Details

Name:	cstm_RepaymentOfObligationsUnderFinanceLeasesAndOtherDebts
Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of outstanding funds that the entity is obligated to repay.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 55
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_55&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_Borrowings
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

The cash inflow from the issuing of bonds, notes and debentures.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 7
-IssueDate 2017-01-01
-Paragraph 17
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=7&date=2017-03-01&anchor=para_17&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_ProceedsFromIssueOfBondsNotesAndDebentures
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow for repayments of bonds, notes and debentures.

+ References

+ Details

Name:	ifrs-full_RepaymentsOfBondsNotesAndDebentures
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash outflow for repayments of non-current borrowings. [Refer: Borrowings]

+ References

+ Details

Name:	ifrs-full_RepaymentsOfNoncurrentBorrowings
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration