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NON-CASH INVESTING AND FINANCING TRANSACTIONS (Details) - EUR (€) € in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Disclosure of cash flow statement [Abstract]
Property, plant and equipment acquired by finance lease	€ 25	€ 33
Fair value of vested PSUs and RSUs	€ 0	€ 4

X

- Definition

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

+ References

No definition available.

+ Details

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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
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X

- Definition

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

+ References

No definition available.

+ Details

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