Consolidated Statement of Changes in Equity - EUR (€) € in Millions	Total	Share capital [member]	Share premium [member]	Re-measurement [member]	Reserve of cash flow hedges and net investment hedges [member]	Foreign currency translation reserve [member]	Other reserves [member]	Retained losses [member]	Total Equity holders of Constellium [member]	Non-controlling interests [member]
Beginning balance at Dec. 31, 2015	€ (540)	€ 2	€ 162	€ (133)		€ 6	€ 11	€ (599)	€ (551)	€ 11
Net income / (loss)	(4)							(4)	(4)
Other comprehensive income / (loss)	(30)			(18)	€ (18)	6			(30)
Total comprehensive income / (loss)	(34)			(18)	(18)	6		(4)	(34)
Transactions with equity holders Share-based compensation	6						6		6
Transactions with non-controlling interests	(2)									(2)
Ending balance at Dec. 31, 2016	(570)	2	162	(151)	(18)	12	17	(603)	(579)	9
Net income / (loss)	(31)							(31)	(31)
Other comprehensive income / (loss)	15			4	31	(19)			16	(1)
Total comprehensive income / (loss)	(16)			4	31	(19)		(31)	(15)	(1)
Share issuance	259	1	258						259
Transactions with equity holders Share-based compensation	8						8		8
Ending balance (Restated [member]) at Dec. 31, 2017	(321)	3	420	(147)	13	(7)	25	(636)	(329)	8
Ending balance at Dec. 31, 2017	(319)	3	420	(147)	13	(7)	25	(634)	(327)	8
Change in accounting policies ending balance at Dec. 31,2018 \| Change in accounting policies [member]	(2)							(2)	(2)
Net income / (loss)	190							188	188	2
Other comprehensive income / (loss)	7			18	(21)	10			7
Total comprehensive income / (loss)	197			18	(21)	10		188	195	2
Share issuance		0	0
Transactions with equity holders Share-based compensation	12						12		12
Transactions with non-controlling interests	(2)									(2)
Ending balance at Dec. 31, 2018	€ (114)	€ 3	€ 420	€ (129)	€ (8)	€ 3	€ 37	€ (448)	€ (122)	€ 8

Transaction with noncontrolling interests.

No definition available.

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

- Definition

The increase (decrease) in equity. [Refer: Equity]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 106
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_106_d&doctype=Standard
-URIDate 2018-03-16

+ Details

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

- Definition

The amount of change in equity resulting from transactions and other events, other than those changes resulting from transactions with owners in their capacity as owners.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 1
-IssueDate 2018-01-01
-Paragraph 32
-Subparagraph a
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=1&date=2018-03-01&anchor=para_32_a_ii&doctype=Standard
-URIDate 2018-03-16

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 1
-IssueDate 2018-01-01
-Paragraph 24
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=1&date=2018-03-01&anchor=para_24_b&doctype=Standard
-URIDate 2018-03-16

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 12
-IssueDate 2018-01-01
-Paragraph B12
-Subparagraph b
-Clause ix
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=12&date=2018-03-01&anchor=para_B12_b_ix&doctype=Appendix&seq=2
-URIDate 2018-03-16

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 106
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_106_a&doctype=Standard
-URIDate 2018-03-16

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Name IFRS
-Number 12
-IssueDate 2018-01-01
-Paragraph B10
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=12&date=2018-03-01&anchor=para_B10_b&doctype=Appendix&seq=2
-URIDate 2018-03-16

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 81A
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_81A_c&doctype=Standard
-URIDate 2018-03-16

+ Details

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

- Definition

The amount of residual interest in the assets of the entity after deducting all its liabilities.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 1
-IssueDate 2018-01-01
-Paragraph 24
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=1&date=2018-03-01&anchor=para_24_a&doctype=Standard
-URIDate 2018-03-16

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 1
-IssueDate 2018-01-01
-Paragraph 32
-Subparagraph a
-Clause i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=1&date=2018-03-01&anchor=para_32_a_i&doctype=Standard
-URIDate 2018-03-16

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 13
-IssueDate 2018-01-01
-Paragraph 93
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=13&date=2018-03-01&anchor=para_93_a&doctype=Standard
-URIDate 2018-03-16

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 13
-IssueDate 2018-01-01
-Paragraph 93
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=13&date=2018-03-01&anchor=para_93_b&doctype=Standard
-URIDate 2018-03-16

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 13
-IssueDate 2018-01-01
-Paragraph 93
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=13&date=2018-03-01&anchor=para_93_e&doctype=Standard
-URIDate 2018-03-16

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 55
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_55&doctype=Standard
-URIDate 2018-03-16

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 78
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_78_e&doctype=Standard
-URIDate 2018-03-16

+ Details

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

- Definition

The increase (decrease) in equity resulting from share-based payment transactions. [Refer: Equity]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 106
-Subparagraph d
-Clause iii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_106_d_iii&doctype=Standard
-URIDate 2018-03-16

+ Details

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

- Definition

The increase in equity through the issue of equity instruments.

+ References

+ Details

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

- Definition

The amount of income and expense (including reclassification adjustments) that is not recognised in profit or loss as required or permitted by IFRSs. [Refer: IFRSs [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 106
-Subparagraph d
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_106_d_ii&doctype=Standard
-URIDate 2018-03-16

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 12
-IssueDate 2018-01-01
-Paragraph B12
-Subparagraph b
-Clause viii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=12&date=2018-03-01&anchor=para_B12_b_viii&doctype=Appendix&seq=2
-URIDate 2018-03-16

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 81A
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_81A_b&doctype=Standard
-URIDate 2018-03-16

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 91
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_91_a&doctype=Standard
-URIDate 2018-03-16

+ Details

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

- Definition

The total of income less expenses from continuing and discontinued operations, excluding the components of other comprehensive income. [Refer: Other comprehensive income]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 7
-IssueDate 2018-01-01
-Paragraph 18
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=7&date=2018-03-01&anchor=para_18_b&doctype=Standard
-URIDate 2018-03-16

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 1
-IssueDate 2018-01-01
-Paragraph 24
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=1&date=2018-03-01&anchor=para_24_b&doctype=Standard
-URIDate 2018-03-16

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 1
-IssueDate 2018-01-01
-Paragraph 32
-Subparagraph a
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=1&date=2018-03-01&anchor=para_32_a_ii&doctype=Standard
-URIDate 2018-03-16

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2018-01-01
-Paragraph 28
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=8&date=2018-03-01&anchor=para_28_b&doctype=Standard
-URIDate 2018-03-16

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Note Effective on first application of IFRS 9
-Name IFRS
-Number 4
-IssueDate 2018-01-01
-Paragraph 39L
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=4&date=2018-03-02&anchor=para_39L_e&doctype=Standard&book=b
-URIDate 2018-03-16

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2018-01-01
-Paragraph 23
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=8&date=2018-03-01&anchor=para_23&doctype=Standard
-URIDate 2018-03-16

Reference 7: http://www.xbrl.org/2003/role/exampleRef
-Name IFRS
-Number 12
-IssueDate 2018-01-01
-Paragraph B10
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=12&date=2018-03-01&anchor=para_B10_b&doctype=Appendix&seq=2
-URIDate 2018-03-16

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 106
-Subparagraph d
-Clause i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_106_d_i&doctype=Standard
-URIDate 2018-03-16

Reference 9: http://www.xbrl.org/2003/role/exampleRef
-Note Effective 2021-01-01
-Name IFRS
-Number 17
-IssueDate 2018-01-01
-Paragraph 113
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=17&date=2018-03-01&anchor=para_113_b&doctype=Standard
-URIDate 2018-03-16

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 81A
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_81A_a&doctype=Standard
-URIDate 2018-03-16

+ Details

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

- Details

Name:	ifrs-full_RetrospectiveApplicationAndRetrospectiveRestatementAxis=cstm_RestatedBalanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RetrospectiveApplicationAndRetrospectiveRestatementAxis=ifrs-full_FinancialEffectOfChangesInAccountingPolicyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: