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COVID-19-RELATED GOVERNMENT ASSISTANCE (Details) € in Millions	6 Months Ended
	Jun. 30, 2020 EUR (€)
Government Assistance [Abstract]
Proceeds From Government Assistance	€ 14

X

- Definition

Government Assistance [Abstract]

+ References

No definition available.

+ Details

Name:	cstm_GovernmentAssistanceAbstract
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