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Trade Receivables and Other - Summary of Carrying Amounts of Total Trade Receivables - Net by Currency (Detail) - EUR (€) € in Millions	Dec. 31, 2017	Dec. 31, 2016
Trade receivables [line items]
Total trade receivables-net	€ 306	€ 235
Euro [member]
Trade receivables [line items]
Total trade receivables-net	124	101
U.S. Dollar [member]
Trade receivables [line items]
Total trade receivables-net	164	115
Swiss Franc [member]
Trade receivables [line items]
Total trade receivables-net	4	3
Other currencies [member]
Trade receivables [line items]
Total trade receivables-net	€ 14	€ 16

X

- Definition

Trade receivables [line items]

+ 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 due from customers for goods and services sold.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 78
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_78_b&doctype=Standard
-URIDate 2017-03-09

+ Details

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Namespace Prefix:	ifrs-full_
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Balance Type:	debit
Period Type:	instant

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type: