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Provisions - Summary of Legal Claims and Other Costs (Detail) - Legal claims and other costs [member] - EUR (€) € in Millions	Jun. 30, 2019	Dec. 31, 2018
Disclosure of other provisions [line items]
Litigation	€ 19	€ 45
Disease claims	4	4
Other	5	5
Total Provisions for legal claims and other costs	€ 28	€ 54

X

- Definition

Provision for disease claims.

+ References

No definition available.

+ Details

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Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Provision for litigation.

+ References

No definition available.

+ Details

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Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- 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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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The amount of provision for legal proceedings. [Refer: Other provisions]

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 37
-IssueDate 2018-01-01
-Section Example 10 A court case
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=37&date=2018-03-01&anchor=sect_Example10Acourtcase&doctype=Illustrative%20Examples
-URIDate 2018-03-16

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 37
-IssueDate 2018-01-01
-Paragraph 87
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=37&date=2018-03-01&anchor=para_87&doctype=Standard
-URIDate 2018-03-16

+ Details

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

The amount of miscellaneous other provisions. [Refer: Other provisions]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 78
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_78_d&doctype=Standard
-URIDate 2018-03-16

+ Details

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type: