Deferred Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) - EUR (€) € in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Significant components of deferred tax assets and liabilities [line items]
At January 1	€ 222	€ 260
Recognized in Profit or loss	(54)	(50)
Recognized in OCI	(20)	11
Effect of change in foreign exchange rates	(9)	2
other		(1)
At December 31	139	222
Long-term assets [member]
Significant components of deferred tax assets and liabilities [line items]
At January 1	(90)	(27)
Recognized in Profit or loss	5	(59)
Effect of change in foreign exchange rates	9	(4)
At December 31	(76)	(90)
Inventories [member]
Significant components of deferred tax assets and liabilities [line items]
At January 1	6	4
Recognized in Profit or loss	(2)	3
other		(1)
At December 31	4	6
Pensions [member]
Significant components of deferred tax assets and liabilities [line items]
At January 1	188	193
Recognized in Profit or loss	(39)	(11)
Recognized in OCI	(5)	2
Effect of change in foreign exchange rates	(14)	4
At December 31	130	188
Derivative valuation [member]
Significant components of deferred tax assets and liabilities [line items]
At January 1	13	27
Recognized in Profit or loss	(17)	(21)
Recognized in OCI	(15)	9
Effect of change in foreign exchange rates	(1)
other		(2)
At December 31	(20)	13
Tax losses carried forward [member]
Significant components of deferred tax assets and liabilities [line items]
At January 1	79	40
Recognized in Profit or loss	3	38
Effect of change in foreign exchange rates	(4)	1
At December 31	78	79
Other [member]
Significant components of deferred tax assets and liabilities [line items]
At January 1	26	23
Recognized in Profit or loss	(3)
Effect of change in foreign exchange rates		1
other		2
At December 31	€ 23	€ 26

Deferred tax relating to other items.

No definition available.

Name:	cstm_DeferredTaxRelatingToOtherItems
Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Significant components of deferred tax assets and liabilities line items.

+ References

No definition available.

+ Details

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

- Definition

The amount of tax expense or income relating to changes in deferred tax liabilities and deferred tax assets, recognised in profit or loss. [Refer: Deferred tax assets; Deferred tax expense (income); Deferred tax liabilities]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 12
-IssueDate 2017-01-01
-Paragraph 81
-Subparagraph g
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2017-03-01&anchor=para_81_g_ii&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_DeferredTaxExpenseIncomeRecognisedInProfitOrLoss
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of income tax relating to amounts recognised in other comprehensive income. [Refer: Other comprehensive income]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 12
-IssueDate 2017-01-01
-Paragraph 81
-Subparagraph ab
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2017-03-01&anchor=para_81_ab&doctype=Standard
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 90
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_90&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_IncomeTaxRelatingToComponentsOfOtherComprehensiveIncome
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

The increase (decrease) in deferred tax liability (asset) resulting from the net exchange differences arising when the financial statements are translated from the functional currency into a different presentation currency, including the translation of a foreign operation into the presentation currency of the reporting entity. [Refer: Deferred tax liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 12
-IssueDate 2017-01-01
-Paragraph 81
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2017-03-01&anchor=para_81&doctype=Standard
-URIDate 2017-03-09

+ Details

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

- Definition

The amount of deferred tax assets net of deferred tax liabilities, when the absolute amount of deferred tax assets is greater than the absolute amount of deferred tax liabilities. [Refer: Deferred tax assets; Deferred tax liabilities]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 12
-IssueDate 2017-01-01
-Paragraph 81
-Subparagraph g
-Clause i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2017-03-01&anchor=para_81_g_i&doctype=Standard
-URIDate 2017-03-09

+ Details

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

- Details

Name:	ifrs-full_AssetsAndLiabilitiesAxis=cstm_LongTermAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_AssetsAndLiabilitiesAxis=cstm_InventoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_AssetsAndLiabilitiesAxis=cstm_PensionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_AssetsAndLiabilitiesAxis=cstm_DerivativeValuationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_AssetsAndLiabilitiesAxis=cstm_TaxLossesCarriedForwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_AssetsAndLiabilitiesAxis=cstm_OtherDeferredTaxAssetsAndLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: