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

Disclosure of liquidity risk.

+ References

No definition available.

+ Details

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Namespace Prefix:	cstm_
Data Type:	xbrli:stringItemType
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
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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+ Details

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

- Definition

The amount of current derivative financial assets. [Refer: Derivative financial assets]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2020-01-01
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-URIDate 2020-01-01

+ Details

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

- 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
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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
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+ Details

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

- Details

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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_BorrowingsByNameAxis=cstm_PanUsAblFacilityDueTwoThousandTwentyTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	ifrs-full_BorrowingsByNameAxis=cstm_UnsecuredCreditFacilityGermanyMember
Namespace Prefix:
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FINANCIAL RISK MANAGEMENT - Narrative (Details) $ in Millions	Jun. 30, 2021 EUR (€)	Jun. 30, 2021 USD ($)	Dec. 31, 2020 EUR (€)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 EUR (€)	Dec. 31, 2019 EUR (€)
Disclosure of liquidity risk [line items]
Margin requirement	€ 63,000,000		€ 39,000,000
Revolving credit facilities outstanding availability	461,000,000
Liquidity value	887,000,000
Cash and cash equivalents	290,000,000		439,000,000		€ 378,000,000	€ 184,000,000
Undrawn facilities	597,000,000
Margin call
Disclosure of liquidity risk [line items]
Margin requirement	€ 0		€ 3,000,000
Derivative classified as cash flow hedge
Disclosure of liquidity risk [line items]
Nominal amount hedging instrument \| $		$ 305		$ 330
U.S. Revolving Facility
Disclosure of liquidity risk [line items]
Borrowing base \| $		337
French Inventory Based Facility
Disclosure of liquidity risk [line items]
Borrowing base \| $		82
German Facility
Disclosure of liquidity risk [line items]
Borrowing base \| $		$ 50