Available borrowings under credit facility.

No definition available.

Name:	cstm_BorrowingFacility
Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Derivative instrument maturity date.

+ References

No definition available.

+ Details

Name:	cstm_DerivativeInstrumentMaturityDate
Namespace Prefix:	cstm_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

Total nominal amount of hedging instrument.

+ References

No definition available.

+ Details

Name:	cstm_FullNominalAmountOfHedgingInstrument
Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Revolving credit facility outstanding.

+ References

No definition available.

+ Details

Name:	cstm_RevolvingCreditFacilityOutstanding
Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of cash on hand and demand deposits, along with short-term, highly liquid investments that are readily convertible to known amounts of cash and that are subject to an insignificant risk of changes in value. [Refer: Cash; Cash equivalents]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 7
-IssueDate 2017-01-01
-Paragraph 45
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=7&date=2017-03-01&anchor=para_45&doctype=Standard
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 12
-IssueDate 2017-01-01
-Paragraph B13
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=12&date=2017-03-01&anchor=para_B13_a&doctype=Appendix&seq=2
-URIDate 2017-03-09

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 54
-Subparagraph i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_54_i&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_CashAndCashEquivalents
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

+ References

No definition available.

+ Details

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

- Definition

The amount of financial assets held for managing liquidity risk (for example, financial assets that are readily saleable or expected to generate cash inflows to meet cash outflows on financial liabilities). [Refer: Liquidity risk [member]; Financial assets; Financial liabilities]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph B11E
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_B11E&doctype=Appendix&seq=2
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_FinancialAssetsHeldForManagingLiquidityRisk
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of undrawn borrowing facilities that may be available for future operating activities and to settle capital commitments. [Refer: Capital commitments]

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 7
-IssueDate 2017-01-01
-Paragraph 50
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=7&date=2017-03-01&anchor=para_50_a&doctype=Standard
-URIDate 2017-03-09

+ Details

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

- Details

Name:	ifrs-full_BorrowingsByNameAxis=cstm_PanUsAblFacilityDueTwoThousandTwentyTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_BorrowingsByNameAxis=cstm_SecuredInventoryBasedFacilityDueTwoThousandNineteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_HedgingInstrumentsAxis=cstm_DerivativeClassifiedAsCashFlowHedgeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_BottomOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_TopOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Financial Risk Management - Additional Information (Detail) € in Millions, $ in Millions	6 Months Ended
	Jun. 30, 2018 EUR (€)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 EUR (€)
Disclosure of credit risk [line items]
Liquidity value	€ 561
Cash and cash equivalents	166		€ 269		€ 347
Undrawn facilities	395
Revolving credit facilities outstanding	263
Pan US ABL facility (due 2022) [member]
Disclosure of credit risk [line items]
Borrowing base \| $		$ 292
Secured inventory based facility (due 2019) [member]
Disclosure of credit risk [line items]
Borrowing base	€ 90
Derivative classified as cash flow hedge [member]
Disclosure of credit risk [line items]
Nominal amount hedging instrument \| $		$ 427		$ 484
Bottom of range [member] \| Derivative classified as cash flow hedge [member]
Disclosure of credit risk [line items]
Maturity period	2018
Top of range [member] \| Derivative classified as cash flow hedge [member]
Disclosure of credit risk [line items]
Maturity period	2022