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Borrowings - Summary of Reconciliation of Movement in Net Borrowings Explanatory (Detail) - EUR (€) € in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Disclosure of detailed information about borrowings [abstract]
Beginning Balance	€ 2,151	€ 2,127	€ 2,127
IFRS 16 application	102
At January 1	2,253
Proceeds / (Repayments) from Revolving Credit Facilities and other loans	76		(68)
Payment of lease liabilities	(70)	€ (7)	(15)
Borrowings assumed through business combination	75
New leases and other loans	33		28
Movement in interests accrued	1		12
Deferred arrangement fees	2		2
Effects of changes in foreign exchange rates	8		65
Ending Balance	€ 2,378		€ 2,151

X

- Definition

The effect of changes in foreign exchange rates on borrowings.

+ References

No definition available.

+ Details

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

X

- Definition

Borrowings IFRS 16 application

+ References

No definition available.

+ Details

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

X

- Definition

Borrowings, restated

+ References

No definition available.

+ Details

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Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Deferred arrangement fees, step-up amortization and other.

+ References

No definition available.

+ Details

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Namespace Prefix:	cstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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X

- Definition

Loans and leases receivables other.

+ References

No definition available.

+ Details

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

X

- Definition

Movement in accrued interest and interest cost capitalized.

+ References

No definition available.

+ Details

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

X

- Definition

The amount of outstanding funds that the entity is obligated to repay.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-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

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Balance Type:	credit
Period Type:	instant

X

- References

No definition available.

+ Details

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X

- Definition

The increase in regulatory deferral account credit balances resulting from items assumed in a business combination. [Refer: Business combinations [member]; Regulatory deferral account credit balances]

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IFRS
-Number 14
-IssueDate 2018-01-01
-Paragraph 33
-Subparagraph a
-Clause iii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=14&date=2018-03-01&anchor=para_33_a_iii&doctype=Standard
-URIDate 2018-03-16

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X

- Definition

The cash outflow for payment of lease liabilities, classified as financing activities. [Refer: Lease liabilities]

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Note Effective 2019-01-01
-Name IAS
-Number 7
-IssueDate 2018-01-01
-Paragraph 17
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=7&date=2018-03-01&anchor=para_17_e&doctype=Standard
-URIDate 2018-03-16

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X

- Definition

The cash outflow for repayments of non-current borrowings. [Refer: Borrowings]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 7
-IssueDate 2018-01-01
-Paragraph 17
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=7&date=2018-03-01&anchor=para_17&doctype=Standard
-URIDate 2018-03-16

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