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DEFERRED INCOME TAXES - Narrative (Details) - EUR (€) € in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Abstract]
Net deferred tax asset balance	€ 183	€ 161	€ 141
Deductible temporary differences and unused tax losses for which no deferred tax asset recognized	920	1,009
Related tax impact	€ 224	€ 259

X

- Definition

Deductible Temporary Differences And Unused Tax Losses For Which No Deferred Tax Asset Is Recognised

+ References

No definition available.

+ Details

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X

- Definition

Income Taxes [Abstract]

+ References

No definition available.

+ Details

Name:	cstm_IncomeTaxesAbstract
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X

- Definition

The amount of deductible temporary differences for which no deferred tax asset is recognised in the statement of financial position. [Refer: Temporary differences [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 12
-IssueDate 2020-01-01
-Paragraph 81
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=12&code=ifrs-tx-2020-en-r&anchor=para_81_e&doctype=Standard
-URIDate 2020-01-01

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X

- Definition

The amount of deferred tax liabilities or assets. [Refer: Deferred tax liabilities; Deferred tax assets]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 12
-IssueDate 2020-01-01
-Paragraph 81
-Subparagraph g
-Clause i
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=12&code=ifrs-tx-2020-en-r&anchor=para_81_g_i&doctype=Standard
-URIDate 2020-01-01

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