Number of other equity instruments vested in share-based payment arrangement.

No definition available.

Name:	cstm_NumberOfOtherEquityInstrumentsVestedInSharebasedPaymentArrangement
Namespace Prefix:	cstm_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of shares in other equity instruments (ie other than share options) granted in a share-based payment arrangement.

+ References

No definition available.

+ Details

Name:	cstm_NumberOfSharesInOtherEquityInstrumentsGrantedInSharebasedPaymentArrangement
Namespace Prefix:	cstm_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of sharebased payments vesting.

+ References

No definition available.

+ Details

Name:	cstm_PercentageOfSharebasedPaymentsVesting
Namespace Prefix:	cstm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of sharebased payments vesting multiplier.

+ References

No definition available.

+ Details

Name:	cstm_PercentageOfSharebasedPaymentsVestingMultiplier
Namespace Prefix:	cstm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Potential additional shares to be vested.

+ References

No definition available.

+ Details

Name:	cstm_PotentialAdditionalSharesToBeVested
Namespace Prefix:	cstm_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Share based compensation measurement period.

+ References

No definition available.

+ Details

Name:	cstm_ShareBasedCompensationMeasurementPeriod
Namespace Prefix:	cstm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Share based compensation vesting period.

+ References

No definition available.

+ Details

Name:	cstm_ShareBasedCompensationVestingPeriod
Namespace Prefix:	cstm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Share based Payment Arrangement Vesting Period

+ References

No definition available.

+ Details

Name:	cstm_ShareBasedPaymentArrangementVestingPeriod
Namespace Prefix:	cstm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Share based payment plans.

+ References

No definition available.

+ Details

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

- Definition

Sharebased Compensation stockholders return percentage.

+ References

No definition available.

+ Details

Name:	cstm_SharebasedCompensationStockholdersReturnPercentage
Namespace Prefix:	cstm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The description of the maximum term of options granted for a type of share-based payment arrangement that existed at any time during the period. An entity with substantially similar types of share-based payment arrangements may aggregate this information. [Refer: Share-based payment arrangements [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 2
-IssueDate 2017-01-01
-Paragraph 45
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=2&date=2017-03-01&anchor=para_45_a&doctype=Standard
-URIDate 2017-03-09

+ Details

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

- Definition

The description of the vesting requirements for a type of share-based payment arrangement that existed at any time during the period. An entity with substantially similar types of share-based payment arrangements may aggregate this information. [Refer: Share-based payment arrangements [member]]

+ References

+ Details

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

- Definition

The amount of expense from share-based payment transactions with employees.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_112_c&doctype=Standard
-URIDate 2017-03-09

+ Details

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

- Details

Name:	ifrs-full_MaturityAxis=cstm_FirstTestingPeriodMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=cstm_PerformanceShareUnitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_BottomOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_TopOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=cstm_PerformanceShareUnitsGrantedInNovemberTwoThousandAndFifteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=cstm_PerformanceShareUnitsGrantedInMarchTwoThousandAndSixteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=cstm_PerformanceShareUnitsGrantedInAugustTwoThousandAndSixteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=cstm_PerformanceShareUnitsGrantedInNovemberTwoThousandAndSixteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=cstm_RestrictedStockUnitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=cstm_EquityAwardsPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Share-Based Compensation - Additional Information (Detail) € in Millions	12 Months Ended
	Dec. 31, 2017 EUR (€) shares	Dec. 31, 2016 EUR (€) shares	Dec. 31, 2015 EUR (€)
Share Based Payment Plans [Line Items]
Total expense related to the potential ordinary shares \| €	€ 8	€ 6	€ 5
PSU granted in 2016 first testing period [member]
Share Based Payment Plans [Line Items]
Potential additional shares to be vested	186,059
Vesting date of additional shares	2019
Performance share units (PSUs) [member]
Share Based Payment Plans [Line Items]
Share based compensation, vesting period	3 years
Percentage of vesting multiplier	100.00%
Share based compensation, measurement period	3 years
Performance share units (PSUs) [member] \| Bottom of range [member]
Share Based Payment Plans [Line Items]
Percentage of vesting multiplier		0.00%	0.00%
Percentage of vesting	0.00%
Performance share units (PSUs) [member] \| Top of range [member]
Share Based Payment Plans [Line Items]
Percentage of vesting multiplier		300.00%	300.00%
Percentage of vesting	200.00%
Performance share units granted in November 2015 [member]
Share Based Payment Plans [Line Items]
Total Stockholders return percentage achieved	251.10%	118.20%
Potential additional shares to be vested	366,669	47,229
Vesting date of additional shares	November 2018	November 2018
Performance share units granted in March 2016 [member] \| PSU granted in 2016 first testing period [member]
Share Based Payment Plans [Line Items]
Total Stockholders return percentage achieved	115.90%
Performance share units granted in August 2016 [member] \| PSU granted in 2016 first testing period [member]
Share Based Payment Plans [Line Items]
Total Stockholders return percentage achieved	191.60%
Performance share units granted in November 2016(member) \| PSU granted in 2016 first testing period [member]
Share Based Payment Plans [Line Items]
Total Stockholders return percentage achieved	223.80%
Restricted Stock Units [member]
Share Based Payment Plans [Line Items]
Vesting period on the grant date	3 years
Number of RSU granted		150,000
Description of vesting requirements for share-based payment arrangement		In 2016, the Company also granted 150,000 RSUs which vest in equal installments on the first two anniversaries of the grant date, subject to continued employment, of which 75,000 vested on the first anniversary in 2017.
Number of RSU vested	75,000
Equity Awards Plans [member]
Share Based Payment Plans [Line Items]
Description of vesting requirements for share-based payment arrangement	Company Board members have been granted RSU awards annually since 2012. These RSUs vest in equal installments on the first two anniversaries of the date of grant, subject to their continued service.