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Trade Payables and Other - Detailed information of contract liabilities (Detail) - EUR (€) € in Millions	Jun. 30, 2019	Dec. 31, 2018
Disclosure of disaggregation of contract liabilities with customers [line items]
Non-current	€ 11	€ 9
Current	59	68
Deferred Tooling Revenue [member]
Disclosure of disaggregation of contract liabilities with customers [line items]
Non-current	5
Advance Payment From Customers [member]
Disclosure of disaggregation of contract liabilities with customers [line items]
Non-current	4	7
Current	6	9
Unrecognized Variable Consideration [member]
Disclosure of disaggregation of contract liabilities with customers [line items]
Non-current	2	2
Current	52	57
Prepayment [member]
Disclosure of disaggregation of contract liabilities with customers [line items]
Current	€ 1	€ 2

X

- Definition

Line items representing disaggregation of contract liabilities with customers.

+ References

No definition available.

+ Details

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Namespace Prefix:	cstm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The amount of current contract liabilities. [Refer: Contract liabilities]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 15
-IssueDate 2018-01-01
-Paragraph 105
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=15&date=2018-03-01&anchor=para_105&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 non-current contract liabilities. [Refer: Contract liabilities]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 15
-IssueDate 2018-01-01
-Paragraph 105
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=15&date=2018-03-01&anchor=para_105&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:
Data Type:	na
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- Details

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Namespace Prefix:
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Balance Type:
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- Details

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Namespace Prefix:
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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: