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Share Capital - Additional Information (Detail) | Jun. 30, 2018 EUR (€) € / shares shares
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Disclosure of classes of share capital [abstract] |
Authorized share capital | € | € 8,000,000
Number of share authorized | shares | 400,000,000
Nominal value of per shares | € / shares | € 0.02

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- Definition Authorised capital. + References No definition available. + Details
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Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
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Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The number of shares authorised. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2017-01-01 -Paragraph 79 -Subparagraph a -Clause i -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_79_a_i&doctype=Standard -URIDate 2017-03-09 + Details
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Namespace Prefix: | ifrs-full_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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- Definition The nominal value per share. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2017-01-01 -Paragraph 79 -Subparagraph a -Clause iii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_79_a_iii&doctype=Standard -URIDate 2017-03-09 + Details
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Namespace Prefix: | ifrs-full_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant