Number of auto-controlled shares.

No definition available.

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Namespace Prefix:	cstm_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Share issue price.

+ References

No definition available.

+ Details

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Namespace Prefix:	cstm_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- 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

- Definition

The increase (decrease) in the number of ordinary shares issued. [Refer: Ordinary shares [member]]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_112_c&doctype=Standard
-URIDate 2017-03-09

+ Details

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash inflow from the issuing of ordinary shares. [Refer: Ordinary shares [member]]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 7
-IssueDate 2017-01-01
-Paragraph 17
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=7&date=2017-03-01&anchor=para_17&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

- Definition

The amount of cost related to the issuance of shares.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 106
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_106_d&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

- Details

Name:	ifrs-full_ClassesOfShareCapitalAxis=ifrs-full_OrdinarySharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_DefinedBenefitPlansAxis=cstm_EmployeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfShareCapitalAxis=cstm_CommonSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_DefinedBenefitPlansAxis=cstm_BoardMembersMember
Namespace Prefix:
Data Type:	na
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Share Capital - Schedule of Share Capital (Parenthetical) (Detail) € in Millions			12 Months Ended
	Nov. 03, 2017 EUR (€) shares	Nov. 03, 2017 $ / shares	Dec. 31, 2017 EUR (€) shares
Disclosure of classes of share capital [line items]
Share issued			28,928,950
Proceeds from issue of ordinary shares \| €			€ 259
Ordinary shares [member]
Disclosure of classes of share capital [line items]
Share issued	28,750,000
Share issued price per share \| $ / shares		$ 11.00
Proceeds from issue of ordinary shares \| €	€ 259
Share issuance, arrangement fees \| €	€ 12
Treasury shares held			38,597
Employees [member] \| Common shares [member]
Disclosure of classes of share capital [line items]
Share issued			125,000
Board members [member] \| Common shares [member]
Disclosure of classes of share capital [line items]
Share issued			53,950