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v3.10.0.1

Revenue - Additional Information (Detail) | 6 Months Ended
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Jun. 30, 2018
Disclosure of geographical areas [abstract] |
Percentage of payment option over total revenue | 1.00%

X
- Definition Percentage Revenues not recognized at a point in time. + References No definition available. + Details
Name: | cstm_PercentageRevenuesNotRecognizedAtPointInTime
Namespace Prefix: | cstm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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