XML 144 R102.htm IDEA: XBRL DOCUMENT
v3.19.1

Earnings Per Share - Summary of Number of Shares Attributable to Equity Holders (Parenthetical) (Detail) - shares | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Weighted average ordinary shares and adjusted weighted average ordinary shares [abstract] |  |
Potential Ordinary shares | 3,291,875 | 411,902

X
- Definition Calculation of diluted potential ordinary shares. + References No definition available. + Details
Name: | cstm_CalculationOfDilutedPotentialOrdinaryShares
Namespace Prefix: | cstm_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | ifrs-full_WeightedAverageSharesAndAdjustedWeightedAverageSharesAbstract
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration