INTANGIBLE ASSETS (INCLUDING GOODWILL) - Changes in Intangible Assets and Goodwill (Details) - EUR (€) € in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Changes in goodwill [abstract]
Goodwill, beginning balance	€ 422	€ 403
Acquired through business combination	24
Effects of changes in foreign exchange rates	9	19
Goodwill, ending balance	455	422
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	70	68
Acquired through business combination	0
Additions	9	11
Amortization expense	(10)	(13)
Transfer during the year	0	0
Effects of changes in foreign exchange rates	1	4
Intangible assets (excluding goodwill), ending balance	70	70
Cost
Changes in goodwill [abstract]
Goodwill, beginning balance	422
Goodwill, ending balance	455	422
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	204
Intangible assets (excluding goodwill), ending balance	217	204
Less accumulated depreciation and impairment
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	(134)
Intangible assets (excluding goodwill), ending balance	(147)	(134)
Technology
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	22	24
Acquired through business combination	0
Additions	0	0
Amortization expense	(1)	(3)
Transfer during the year	0	0
Effects of changes in foreign exchange rates	0	1
Intangible assets (excluding goodwill), ending balance	21	22
Technology \| Cost
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	84
Intangible assets (excluding goodwill), ending balance	87	84
Technology \| Less accumulated depreciation and impairment
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	(62)
Intangible assets (excluding goodwill), ending balance	(66)	(62)
Computer Software
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	18	18
Acquired through business combination	0
Additions	1	2
Amortization expense	(8)	(8)
Transfer during the year	7	5
Effects of changes in foreign exchange rates	1	1
Intangible assets (excluding goodwill), ending balance	19	18
Computer Software \| Cost
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	65
Intangible assets (excluding goodwill), ending balance	73	65
Computer Software \| Less accumulated depreciation and impairment
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	(47)
Intangible assets (excluding goodwill), ending balance	(54)	(47)
Customer relationships
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	15	15
Acquired through business combination	0
Additions	0	0
Amortization expense	(1)	(1)
Transfer during the year	0	0
Effects of changes in foreign exchange rates	0	1
Intangible assets (excluding goodwill), ending balance	14	15
Customer relationships \| Cost
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	39
Intangible assets (excluding goodwill), ending balance	39	39
Customer relationships \| Less accumulated depreciation and impairment
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	(24)
Intangible assets (excluding goodwill), ending balance	(25)	(24)
Work in Progress
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	13	9
Acquired through business combination	0
Additions	8	8
Amortization expense	0	0
Transfer during the year	(7)	(5)
Effects of changes in foreign exchange rates	0	1
Intangible assets (excluding goodwill), ending balance	14	13
Work in Progress \| Cost
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	13
Intangible assets (excluding goodwill), ending balance	16	13
Work in Progress \| Less accumulated depreciation and impairment
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	0
Intangible assets (excluding goodwill), ending balance	(2)	0
Other
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	2	2
Acquired through business combination	0
Additions	0	1
Amortization expense	0	(1)
Transfer during the year	0	0
Effects of changes in foreign exchange rates	0	0
Intangible assets (excluding goodwill), ending balance	2	2
Other \| Cost
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	3
Intangible assets (excluding goodwill), ending balance	2	3
Other \| Less accumulated depreciation and impairment
Changes in intangible assets other than goodwill [abstract]
Intangible assets (excluding goodwill), beginning balance	(1)
Intangible assets (excluding goodwill), ending balance	€ 0	€ (1)

The increase in intangible assets other than goodwill resulting from acquisitions through business combinations. [Refer: Business combinations [member]; Intangible assets other than goodwill]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 38
-IssueDate 2019-01-01
-Paragraph 118
-Subparagraph e
-Clause i
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=38&code=ifrs-tx-2019-en-r&anchor=para_118_e_i&doctype=Standard
-URIDate 2019-03-27

+ Details

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

- Definition

The amount of additional goodwill recognised, except goodwill included in a disposal group that, on acquisition, meets the criteria to be classified as held for sale in accordance with IFRS 5. [Refer: Goodwill; Disposal groups classified as held for sale [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 3
-IssueDate 2019-01-01
-Paragraph B67
-Subparagraph d
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=3&code=ifrs-tx-2019-en-r&anchor=para_B67_d_ii&doctype=Appendix&subtype=B
-URIDate 2019-03-27

+ Details

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

- Definition

The amount of additions to intangible assets other than goodwill, other than those acquired through business combinations. [Refer: Business combinations [member]; Intangible assets other than goodwill]

+ References

+ Details

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

- Definition

The amount of amortisation of intangible assets other than goodwill. [Refer: Depreciation and amortisation expense; Intangible assets other than goodwill]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 38
-IssueDate 2019-01-01
-Paragraph 118
-Subparagraph e
-Clause vi
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=38&code=ifrs-tx-2019-en-r&anchor=para_118_e_vi&doctype=Standard
-URIDate 2019-03-27

+ Details

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

- References

No definition available.

+ Details

Name:	ifrs-full_ChangesInGoodwillAbstract
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	ifrs-full_ChangesInIntangibleAssetsOtherThanGoodwillAbstract
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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
-Name IFRS
-Number 3
-IssueDate 2019-01-01
-Paragraph B67
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=3&code=ifrs-tx-2019-en-r&anchor=para_B67_d&doctype=Appendix&subtype=B
-URIDate 2019-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 36
-IssueDate 2019-01-01
-Paragraph 134
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=36&code=ifrs-tx-2019-en-r&anchor=para_134_a&doctype=Standard
-URIDate 2019-03-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 36
-IssueDate 2019-01-01
-Paragraph 135
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=36&code=ifrs-tx-2019-en-r&anchor=para_135_a&doctype=Standard
-URIDate 2019-03-27

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 54
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_54_c&doctype=Standard
-URIDate 2019-03-27

+ Details

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

- Definition

The increase (decrease) in goodwill resulting from net exchange differences arising on the translation of the financial statements 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: Goodwill]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 3
-IssueDate 2019-01-01
-Paragraph B67
-Subparagraph d
-Clause vi
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=3&code=ifrs-tx-2019-en-r&anchor=para_B67_d_vi&doctype=Appendix&subtype=B
-URIDate 2019-03-27

+ Details

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

- Definition

The increase (decrease) in intangible assets other than goodwill resulting from net exchange differences arising on the translation of the financial statements 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: Intangible assets other than goodwill]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 38
-IssueDate 2019-01-01
-Paragraph 118
-Subparagraph e
-Clause vii
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=38&code=ifrs-tx-2019-en-r&anchor=para_118_e_vii&doctype=Standard
-URIDate 2019-03-27

+ Details

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

- Definition

The increase (decrease) in intangible assets and goodwill resulting from transfers. [Refer: Intangible assets and goodwill]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 38
-IssueDate 2019-01-01
-Paragraph 118
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=38&code=ifrs-tx-2019-en-r&anchor=para_118_e&doctype=Standard
-URIDate 2019-03-27

+ Details

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

- Definition

The amount of identifiable non-monetary assets without physical substance. This amount does not include goodwill. [Refer: Goodwill]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 38
-IssueDate 2019-01-01
-Paragraph 118
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=38&code=ifrs-tx-2019-en-r&anchor=para_118_e&doctype=Standard
-URIDate 2019-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 54
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_54_c&doctype=Standard
-URIDate 2019-03-27

+ Details

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

- Details

Name:	ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis=ifrs-full_GrossCarryingAmountMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis=ifrs-full_AccumulatedDepreciationAmortisationAndImpairmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfIntangibleAssetsAndGoodwillAxis=ifrs-full_TechnologybasedIntangibleAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfIntangibleAssetsAndGoodwillAxis=ifrs-full_ComputerSoftwareMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfIntangibleAssetsAndGoodwillAxis=cstm_CustomerRelationships1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfIntangibleAssetsAndGoodwillAxis=cstm_WorkInProgressMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfIntangibleAssetsAndGoodwillAxis=ifrs-full_OtherIntangibleAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: