XML 205 R168.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Share-Based Compensation - Summary of Weighted-Average Fair Value (Detail) | 12 Months Ended
--- | ---
Dec. 31, 2017 EUR (€) shares | Dec. 31, 2016 EUR (€) shares
Performance-Based RSU [member] |  |
Disclosure of movement of potential shares [line items] |  |
Potential Shares, Beginning balance | shares | 2,066,835 | 1,007,000
Potential Shares, Granted | shares | 892,781 | 1,292,000
Potential Shares, Over performance | shares | 552,728 | 47,229
Potential Shares, Forfeited | shares | (254,504) | (279,394)
Potential Shares, Ending balance | shares | 3,257,840 | 2,066,835
Weighted- Average Grant-Date Fair Value per Share, Beginning balance | € | € 7.50 | € 7.77
Weighted- Average Grant-Date Fair Value per Share, Granted | € | 11.52 | 7.56
Weighted- Average Grant-Date Fair Value per Share, Over performance | € | 7.63 | 7.10
Weighted- Average Grant-Date Fair Value per Share, Forfeited | € | 8.29 | 8.71
Weighted- Average Grant-Date Fair Value per Share, Ending balance | € | € 8.56 | € 7.50
Restricted Stock Units [member] |  |
Disclosure of movement of potential shares [line items] |  |
Potential Shares, Beginning balance | shares | 479,500 | 269,500
Potential Shares, Granted | shares | 703,180 | 340,300
Potential Shares, Vested | shares | (125,000) | (87,300)
Potential Shares, Forfeited | shares | (113,180) | (43,000)
Potential Shares, Ending balance | shares | 944,500 | 479,500
Weighted- Average Grant-Date Fair Value per Share, Beginning balance | € | € 7.82 | € 16.10
Weighted- Average Grant-Date Fair Value per Share, Granted | € | 7.50 | 5.40
Weighted- Average Grant-Date Fair Value per Share, Vested | € | 7.02 | 19.75
Weighted- Average Grant-Date Fair Value per Share, Forfeited | € | 7.27 | 16.40
Weighted- Average Grant-Date Fair Value per Share, Ending balance | € | € 7.76 | € 7.82
Equity Awards Plans [member] |  |
Disclosure of movement of potential shares [line items] |  |
Potential Shares, Beginning balance | shares | 94,881 | 34,865
Potential Shares, Granted | shares | 54,409 | 81,858
Potential Shares, Vested | shares | (53,950) | (21,842)
Potential Shares, Ending balance | shares | 95,340 | 94,881
Weighted- Average Grant-Date Fair Value per Share, Beginning balance | € | € 5.01 | € 14.11
Weighted- Average Grant-Date Fair Value per Share, Granted | € | 6.09 | 4.02
Weighted- Average Grant-Date Fair Value per Share, Vested | € | 5.76 | 15.84
Weighted- Average Grant-Date Fair Value per Share, Ending balance | € | € 5.20 | € 5.01

X
- Definition Disclosure of movement of potential shares. + References No definition available. + Details
Name: | cstm_DisclosureOfMovementOfPotentialSharesLineItems
Namespace Prefix: | cstm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of instruments other equity instruments over performance. + References No definition available. + Details
Name: | cstm_NumberOfInstrumentsOtherEquityInstrumentsOverPerformance
Namespace Prefix: | cstm_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of instruments other equity instruments vested. + References No definition available. + Details
Name: | cstm_NumberOfInstrumentsOtherEquityInstrumentsVested
Namespace Prefix: | cstm_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Weighted Average Fair Value On Measurement Date Other Equity Instruments Granted Per Share + References No definition available. + Details
Name: | cstm_WeightedAverageFairValueOnMeasurementDateOtherEquityInstrumentsGrantedPerShare
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

X
- Definition Weighted average grant date fair value per share other equity instruments forfeited. + References No definition available. + Details
Name: | cstm_WeightedAverageGrantDateFairValuePerShareOtherEquityInstrumentsForfeited
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- Definition Weighted average grant date fair value per share other equity instruments, granted. + References No definition available. + Details
Name: | cstm_WeightedAverageGrantDateFairValuePerShareOtherEquityInstrumentsGranted
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- Definition Weighted average grant date fair value per share other equity instruments over performance. + References No definition available. + Details
Name: | cstm_WeightedAverageGrantDateFairValuePerShareOtherEquityInstrumentsOverPerformance
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- Definition Weighted average grant date fair value per share other equity instruments vested. + References No definition available. + Details
Name: | cstm_WeightedAverageGrantDateFairValuePerShareOtherEquityInstrumentsVested
Namespace Prefix: | cstm_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of other equity instruments (ie other than share options) granted in a share-based payment arrangement. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 2 -IssueDate 2017-01-01 -Paragraph 47 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=2&date=2017-03-01&anchor=para_47_b&doctype=Standard -URIDate 2017-03-09 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Name IFRS -Number 2 -IssueDate 2017-01-01 -Paragraph 45 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=2&date=2017-03-01&anchor=para_45&doctype=Standard -URIDate 2017-03-09 + Details
Name: | ifrs-full_NumberOfInstrumentsOtherEquityInstrumentsGranted
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of other equity instruments (ie other than share options) forfeited in a share-based payment arrangement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IFRS -Number 2 -IssueDate 2017-01-01 -Paragraph 45 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=2&date=2017-03-01&anchor=para_45&doctype=Standard -URIDate 2017-03-09 + Details
Name: | ifrs-full_NumberOfOtherEquityInstrumentsForfeitedInSharebasedPaymentArrangement
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of other equity instruments (ie other than share options) outstanding in a share-based payment arrangement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IFRS -Number 2 -IssueDate 2017-01-01 -Paragraph 45 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=2&date=2017-03-01&anchor=para_45&doctype=Standard -URIDate 2017-03-09 + Details
Name: | ifrs-full_NumberOfOtherEquityInstrumentsOutstandingInSharebasedPaymentArrangement
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=cstm_PerformanceBasedRestrictedStockUnitsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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