Carrying value.

No definition available.

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- Definition

Cash flow growth rate.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Cash flow projected period.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Cash flows projections range.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
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Period Type:	duration

- Definition

Perpetual growth rate decrease that would result in the recoverable value equaling the carrying value.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Discount rate increase that would result in the recoverable value equaling the carrying value.

+ References

No definition available.

+ Details

Name:	cstm_IncreaseDiscountRate
Namespace Prefix:	cstm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Recoverable value.

+ References

No definition available.

+ Details

Name:	cstm_RecoverableValue
Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Lower shipments in the Automotive Body Sheet US business that would result in the recoverable value equaling the carrying value.

+ References

No definition available.

+ Details

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Namespace Prefix:	cstm_
Data Type:	num:percentItemType
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
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- Definition

The amount of assets representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognised. [Refer: Business combinations [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

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

- Details

Name:	ifrs-full_SegmentsAxis=cstm_PackagingAndAutomotiveRolledProductsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_BottomOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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Namespace Prefix:
Data Type:	na
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- Details

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- Details

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Intangible Assets (Including Goodwill) - Additional Information (Detail) - EUR (€) € in Millions		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]
Goodwill	€ 403	€ 403	€ 457	€ 443
P&ARP [member]
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]
Goodwill	€ 396	396
Cash flow projected period	8 years
Carrying value	€ 1,204	1,204
Recoverable value	€ 1,554	€ 1,554
Increase discount rate		2.25%
Decrease perpetual growth rate		5.00%
Automotive Body Sheet lower shipments		65.00%
P&ARP [member] \| Bottom of range [member]
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]
Cash flow and a long term growth rate	0.00%
Cash flows projections	10.00%
P&ARP [member] \| Top of range [member]
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]
Cash flow and a long term growth rate	1.50%
Cash flows projections	12.00%
A&T [member]
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]
Goodwill	€ 5	€ 5
AS&I [member]
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]
Goodwill	€ 2	€ 2