XML 124 R116.htm IDEA: XBRL DOCUMENT v3.19.2

Share Capital - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2019 EUR (€) Voting_Rights € / shares shares
Disclosure of classes of share capital [abstract]
Authorized share capital | €	€ 8,000,000
Number of share authorized | shares	400,000,000
Nominal value of per shares | € / shares	€ 0.02
Shares, number of voting rights | Voting_Rights	1

X

- Definition

Authorised capital.

+ References

No definition available.

+ Details

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

X

- Definition

Number of voting right on share.

+ References

No definition available.

+ Details

Name:	cstm_NumberOfVotingRightOnShare
Namespace Prefix:	cstm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

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

X

- Definition

The number of shares authorised.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 79
-Subparagraph a
-Clause i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_79_a_i&doctype=Standard
-URIDate 2018-03-16

+ Details

Name:	ifrs-full_NumberOfSharesAuthorised
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

X

- Definition

The nominal value per share.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 79
-Subparagraph a
-Clause iii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_79_a_iii&doctype=Standard
-URIDate 2018-03-16

+ Details

Name:	ifrs-full_ParValuePerShare
Namespace Prefix:	ifrs-full_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant