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NON-CASH INVESTING AND FINANCING TRANSACTIONS (Details) - EUR (€) € in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of cash flow statement [Abstract]
Property, plant and equipment acquired by finance lease	€ 66	€ 75	€ 28
Fair value of vested PSUs and RSUs	€ 14	€ 8	€ 8

X

- Definition

Additions through noncash finance lease, property, plant and equipment.

+ References

No definition available.

+ Details

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Balance Type:	debit
Period Type:	duration

X

- Definition

Disclosure of cash flow statement

+ References

No definition available.

+ Details

Name:	cstm_DisclosureofcashflowstatementAbstract
Namespace Prefix:	cstm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Fair value of performance stock units and restricted stock units vested.

+ References

No definition available.

+ Details

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