XML 169 R127.htm IDEA: XBRL DOCUMENT
v3.19.1

Deferred Income Taxes - Additional Information (Detail) - EUR (€) | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Significant components of deferred tax assets and liabilities [line items] |  |
US tax rate decrease impact in P&L |  | € (16,000,000)
US tax rate decrease impact in OCI |  | (8,000,000)
Unused tax losses for which no deferred tax asset recognized | € 1,257,000,000 | 1,393,000,000
Related tax impact | 321,000,000 | € 356,000,000
Switzerland [member] |  |
Significant components of deferred tax assets and liabilities [line items] |  |
Tax loss carried forwards | € 0 |

X
- Definition Increase (decrease) in deferred tax recognised in profit or loss due to change in rate. + References No definition available. + Details
Name: | cstm_IncreaseDecreaseInDeferredTaxRecognisedInProfitOrLossDueToChangeInRate
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Non recognized tax assets. + References No definition available. + Details
Name: | cstm_NonRecognizedTaxAssets
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Significant components of deferred tax assets and liabilities line items. + References No definition available. + Details
Name: | cstm_SignificantComponentsOfDeferredTaxAssetsAndLiabilitiesLineItems
Namespace Prefix: | cstm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Unused tax losses carried forward. + References No definition available. + Details
Name: | cstm_UnusedTaxLossesCarriedForward
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The increase (decrease) in accumulated deferred tax recognised in other comprehensive income due to change in the tax rate. [Refer: Other comprehensive income] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2018-01-01 -Paragraph 85 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_85&doctype=Standard -URIDate 2018-03-16 + Details
Name: | ifrs-full_IncreaseDecreaseInAccumulatedDeferredTaxRecognisedInOtherComprehensiveIncomeDueToChangeInTaxRate
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The amount of unused tax losses for which no deferred tax asset is recognised in the statement of financial position. [Refer: Unused tax losses [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 12 -IssueDate 2018-01-01 -Paragraph 81 -Subparagraph e -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2018-03-01&anchor=para_81_e&doctype=Standard -URIDate 2018-03-16 + Details
Name: | ifrs-full_UnusedTaxLossesForWhichNoDeferredTaxAssetRecognised
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | ifrs-full_GeographicalAreasAxis=country_CH
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |