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COVID-19-RELATED GOVERNMENT ASSISTANCE (Details) € in Millions	12 Months Ended
	Dec. 31, 2020 EUR (€)
COVID-19
Government Assistance [Line Items]
Proceeds from government assistance	€ 22

X

- Definition

Government Assistance

+ References

No definition available.

+ Details

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

X

- Definition

Proceeds From Government Assistance, COVID-19

+ References

No definition available.

+ Details

Name:	cstm_ProceedsFromGovernmentAssistanceCOVID19
Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Details

Name:	cstm_UnusualOrInfrequentItemOrBoth1Axis=cstm_COVID19PandemicMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: