Borrowings - Summary of Reconciliation of Movement in Net Borrowings Explanatory (Detail) - EUR (€) € in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Disclosure of detailed information about borrowings [abstract]
At January 1,	€ 2,468	€ 2,233
Proceeds from issuance of Senior Notes	1,440	375
Repayments of Senior Notes or PIK Toggle notes	(1,559)	(148)
Proceeds / (Repayments) from U.S. Revolving Credit Facilities and other loans	29	(69)
Arrangement fees payment	(29)	(12)
Finance lease repayment and others	(13)	(10)
Movement in interests accrued or capitalized	(13)	15
New finance leases	17	16
Deferred arrangement fees and step-up amortization	7	(10)
Effects of changes in foreign exchange rates	(220)	78
At December 31,	€ 2,127	€ 2,468

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
Namespace Prefix:	cstm_
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