Available borrowings under credit facility.

No definition available.

Name:	cstm_BorrowingFacility
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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- Definition

Derivative instrument maturity date.

+ References

No definition available.

+ Details

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Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

Disclosure of liquidity risk.

+ References

No definition available.

+ Details

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

Total nominal amount of hedging instrument.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Nominal amount of hedging instruments.

+ References

No definition available.

+ Details

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Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

Realized gain loss of net investment hedge.

+ References

No definition available.

+ Details

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

- Definition

Revolving credit facility outstanding.

+ References

No definition available.

+ Details

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Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- 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 2018-01-01
-Paragraph 45
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=7&date=2018-03-01&anchor=para_45&doctype=Standard
-URIDate 2018-03-16

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-Paragraph B13
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- 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
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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
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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Details

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- Details

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- Details

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Balance Type:
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- Details

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Data Type:	na
Balance Type:
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- Details

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Financial Risk Management - Additional Information (Detail) € in Millions, SFr in Millions, $ in Millions	6 Months Ended
	Jun. 30, 2019 EUR (€)	Jun. 30, 2019 USD ($)	Jun. 30, 2019 EUR (€)	Jun. 30, 2019 CHF (SFr)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 EUR (€)	Dec. 31, 2017 EUR (€)
Disclosure of liquidity risk [line items]
Liquidity value			€ 588.0
Cash and cash equivalents			213.0			€ 164.0	€ 269.0
Undrawn facilities			375.0
Revolving credit facilities outstanding			314.0
Forward contract [member]
Disclosure of liquidity risk [line items]
Total nominal amount forward contracts \| SFr				SFr 174
Realized gain / loss of the net investment hedge	€ (2.8)
Derivative classified as cash flow hedge [member]
Disclosure of liquidity risk [line items]
Nominal amount hedging instrument \| $		$ 296			$ 369
Pan US ABL facility (due 2022) [member]
Disclosure of liquidity risk [line items]
Borrowing base \| $		$ 374
French inventory facility [member]
Disclosure of liquidity risk [line items]
Borrowing base			€ 78.0
Top of range [member] \| Derivative classified as cash flow hedge [member]
Disclosure of liquidity risk [line items]
Maturity period	2022
Bottom of range [member] \| Derivative classified as cash flow hedge [member]
Disclosure of liquidity risk [line items]
Maturity period	2019