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Subsequent Events - Additional Information (Detail) € in Millions | 1 Months Ended
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Jul. 31, 2018 EUR (€)
Disposal of leased assets [member] | Novelis [member] |
Disclosure of non-adjusting events after reporting period [line items] |
Sale of assets | € 200

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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition The cash inflow from the disposal of property, plant and equipment, intangible assets other than goodwill, investment property and other non-current assets. [Refer: Intangible assets other than goodwill; Investment property; Other non-current assets; Property, plant and equipment] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 7 -IssueDate 2017-01-01 -Paragraph 16 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=7&date=2017-03-01&anchor=para_16&doctype=Standard -URIDate 2017-03-09 + Details
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Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | ifrs-full_SignificantInvestmentsInSubsidiariesAxis=cstm_NovelisMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |