XML 501 R72.htm IDEA: XBRL DOCUMENT
v3.20.4

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Disclosure of Foreign Exchange Rates (Details) | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
U.S. Dollars |  |  |
Disclosure of foreign exchange rates [line items] |  |  |
Average rate | 1.1405 | 1.1193 | 1.1798
Closing rate | 1.2271 | 1.1234 | 1.1450
Swiss Francs |  |  |
Disclosure of foreign exchange rates [line items] |  |  |
Average rate | 1.0704 | 1.1121 | 1.1546
Closing rate | 1.0802 | 1.0854 | 1.1269
Czech Koruna |  |  |
Disclosure of foreign exchange rates [line items] |  |  |
Average rate | 26.4337 | 25.6698 | 25.6452
Closing rate | 26.2420 | 25.4080 | 25.7240

X
- Definition Disclosure of foreign exchange rates. + References No definition available. + Details
Name: | cstm_DisclosureOfForeignExchangeRatesLineItems
Namespace Prefix: | cstm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The average exchange rate used by the entity. Exchange rate is the ratio of exchange for two currencies. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2020-01-01 -Paragraph 112 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_112_c&doctype=Standard -URIDate 2020-01-01 + Details
Name: | ifrs-full_AverageForeignExchangeRate
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

X
- Definition The spot exchange rate at the end of the reporting period. Exchange rate is the ratio of exchange for two currencies. Spot exchange rate is the exchange rate for immediate delivery. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2020-01-01 -Paragraph 112 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_112_c&doctype=Standard -URIDate 2020-01-01 + Details
Name: | ifrs-full_ClosingForeignExchangeRate
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | srt_CurrencyAxis=currency_USD
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_CurrencyAxis=currency_CHF
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_CurrencyAxis=currency_CZK
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |