Disclosure of changes in financial position [line items]

No definition available.

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

- Definition

Immediate recognition actuarial gains (losses) arising from change in assumption recognized in profit (loss).

+ References

No definition available.

+ Details

Name:	cstm_ImmediateRecognitionActuarialGainsLossesArisingFromChangeInAssumptionRecognizedInProfitLoss
Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in demographic assumptions that result in remeasurements of the net defined benefit liability (asset). Demographic assumptions deal with matters such as: (a) mortality; (b) rates of employee turnover, disability and early retirement; (c) the proportion of plan members with dependants who will be eligible for benefits; (d) the proportion of plan members who will select each form of payment option available under the plan terms; and (e) claim rates under medical plans. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph c
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_c_ii&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_ActuarialGainsLossesArisingFromChangesInDemographicAssumptionsNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in financial assumptions that result in remeasurements of the net defined benefit liability (asset). Financial assumptions deal with items such as: (a) the discount rate; (b) benefit levels, excluding any cost of the benefits to be met by employees, and future salary; (c) in the case of medical benefits, future medical costs, including claim handling costs (ie the costs that will be incurred in processing and resolving claims, including legal and adjuster's fees); and (d) taxes payable by the plan on contributions relating to service before the reporting date or on benefits resulting from that service. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph c
-Clause iii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_c_iii&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_ActuarialGainsLossesArisingFromChangesInFinancialAssumptionsNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of expenses that the entity classifies as being administrative.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 26
-IssueDate 2017-01-01
-Paragraph 35
-Subparagraph b
-Clause vi
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=26&date=2017-03-01&anchor=para_35_b_vi&doctype=Standard
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 103
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_103&doctype=Standard
-URIDate 2017-03-09

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 99
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_99&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_AdministrativeExpense
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of benefits paid or payable for retirement benefit plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 26
-IssueDate 2017-01-01
-Paragraph 35
-Subparagraph b
-Clause v
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=26&date=2017-03-01&anchor=para_35_b_v&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_BenefitsPaidOrPayable
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from contributions to a defined benefit plan by plan participants. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph f
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_f&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_ContributionsToPlanByPlanParticipantsNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_a&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_CurrentServiceCostNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of decrease in the fair value measurement of liabilities due to a change in one or more unobservable inputs to reflect reasonably possible alternative assumptions.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 13
-IssueDate 2017-01-01
-Paragraph 93
-Subparagraph h
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=13&date=2017-03-01&anchor=para_93_h_ii&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_DecreaseInFairValueMeasurementDueToChangeInOneOrMoreUnobservableInputsToReflectReasonablyPossibleAlternativeAssumptionsLiabilities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of employer contributions to retirement benefit plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 26
-IssueDate 2017-01-01
-Paragraph 35
-Subparagraph b
-Clause i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=26&date=2017-03-01&anchor=para_35_b_i&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_EmployerContributions
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from changes in foreign exchange rates. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_e&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_IncreaseDecreaseThroughChangesInForeignExchangeRatesNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of income or expense arising from interest. [Refer: Interest expense; Interest income]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2017-01-01
-Paragraph 28
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=8&date=2017-03-01&anchor=para_28_e&doctype=Standard
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2017-01-01
-Paragraph 23
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=8&date=2017-03-01&anchor=para_23&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_InterestRevenueExpense
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 140
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_140_a&doctype=Standard
-URIDate 2017-03-09

+ Details

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

- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets. The return on plans assets is interest, dividends and other revenue derived from the plan assets, together with realised and unrealised gains or losses on the plan assets, less any costs of managing plan assets and less any tax payable by the plan itself, other than tax included in the actuarial assumptions used to measure the present value of the defined benefit obligation. [Refer: Plan assets [member]; Net defined benefit liability (asset); Actuarial assumptions [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph c
-Clause i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_c_i&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_ReturnOnPlanAssetsNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	ifrs-full_ConsolidatedAndSeparateFinancialStatementsAxis=cstm_IncomeStatementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	ifrs-full_ConsolidatedAndSeparateFinancialStatementsAxis=ifrs-full_AccumulatedOtherComprehensiveIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ConsolidatedAndSeparateFinancialStatementsAxis=cstm_CashFlowMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_NetDefinedBenefitLiabilityAssetAxis=ifrs-full_PresentValueOfDefinedBenefitObligationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_NetDefinedBenefitLiabilityAssetAxis=ifrs-full_PlanAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Pensions and Other Post-employment Benefit Obligations - Details of Movement in Net Defined Benefit Obligations (Detail) - EUR (€) € in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disclosure of changes in financial position [line items]
Beginning balance			€ 664
Closing balance	€ 643		643
Consolidated Income Statement [member]
Disclosure of changes in financial position [line items]
Current service cost	5	€ 7	11	€ 13
Interest cost / (income)			8
Immediate recognition of gains / (losses) arising over the year			0
Administration expenses	1	0	1	1
Consolidated Income Statement [member] \| Pension benefits [member]
Disclosure of changes in financial position [line items]
Current service cost	3	4	8	9
Administration expenses	1	0	1	1
Consolidated Income Statement [member] \| Other benefits [member]
Disclosure of changes in financial position [line items]
Current service cost	2	€ 3	3	€ 4
Statement of Comprehensive Income (Loss) [member]
Disclosure of changes in financial position [line items]
- actual return less interest on plan assets			9
- changes in financial assumptions			(42)
- changes in demographic assumptions			0
- experience losses			5
Effects of changes in foreign exchange rates			8
Consolidated Statement of Cash Flows [member]
Disclosure of changes in financial position [line items]
Benefits paid			(11)
Contributions by the Group			(10)
Defined benefit obligations [member]
Disclosure of changes in financial position [line items]
Beginning balance			1,051
Closing balance	1,030		1,030
Defined benefit obligations [member] \| Pension benefits [member]
Disclosure of changes in financial position [line items]
Beginning balance			801
Closing balance	789		789
Defined benefit obligations [member] \| Other benefits [member]
Disclosure of changes in financial position [line items]
Beginning balance			250
Closing balance	241		241
Defined benefit obligations [member] \| Consolidated Income Statement [member]
Disclosure of changes in financial position [line items]
Current service cost			11
Interest cost / (income)			12
Immediate recognition of gains / (losses) arising over the year			0
Defined benefit obligations [member] \| Consolidated Income Statement [member] \| Pension benefits [member]
Disclosure of changes in financial position [line items]
Current service cost			8
Interest cost / (income)			8
Immediate recognition of gains / (losses) arising over the year			0
Defined benefit obligations [member] \| Consolidated Income Statement [member] \| Other benefits [member]
Disclosure of changes in financial position [line items]
Current service cost			3
Interest cost / (income)			4
Immediate recognition of gains / (losses) arising over the year			0
Defined benefit obligations [member] \| Statement of Comprehensive Income (Loss) [member]
Disclosure of changes in financial position [line items]
- changes in financial assumptions			(42)
- changes in demographic assumptions			0
- experience losses			5
Effects of changes in foreign exchange rates			16
Defined benefit obligations [member] \| Statement of Comprehensive Income (Loss) [member] \| Pension benefits [member]
Disclosure of changes in financial position [line items]
- changes in financial assumptions			(28)
- changes in demographic assumptions			0
- experience losses			3
Effects of changes in foreign exchange rates			10
Defined benefit obligations [member] \| Statement of Comprehensive Income (Loss) [member] \| Other benefits [member]
Disclosure of changes in financial position [line items]
- changes in financial assumptions			(14)
- changes in demographic assumptions			0
- experience losses			2
Effects of changes in foreign exchange rates			6
Defined benefit obligations [member] \| Consolidated Statement of Cash Flows [member]
Disclosure of changes in financial position [line items]
Benefits paid			(25)
Contributions by the employees			2
Defined benefit obligations [member] \| Consolidated Statement of Cash Flows [member] \| Pension benefits [member]
Disclosure of changes in financial position [line items]
Benefits paid			(15)
Contributions by the employees			2
Defined benefit obligations [member] \| Consolidated Statement of Cash Flows [member] \| Other benefits [member]
Disclosure of changes in financial position [line items]
Benefits paid			(10)
Plan Assets [member]
Disclosure of changes in financial position [line items]
Beginning balance			(387)
Closing balance	€ (387)		(387)
Plan Assets [member] \| Consolidated Income Statement [member]
Disclosure of changes in financial position [line items]
Interest cost / (income)			(4)
Immediate recognition of gains / (losses) arising over the year			0
Administration expenses			1
Plan Assets [member] \| Statement of Comprehensive Income (Loss) [member]
Disclosure of changes in financial position [line items]
- actual return less interest on plan assets			9
- changes in demographic assumptions			0
Effects of changes in foreign exchange rates			(8)
Plan Assets [member] \| Consolidated Statement of Cash Flows [member]
Disclosure of changes in financial position [line items]
Benefits paid			14
Contributions by the Group			(10)
Contributions by the employees			€ (2)