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Deferred tax - Narrative (Details)
€ in Millions
12 Months Ended
Dec. 31, 2017
EUR (€)
subsidiary
Dec. 31, 2016
EUR (€)
Dec. 31, 2015
EUR (€)
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Amount of unused tax losses carried forward € 567.0 € 385.0 € 434.0
Amount of statutory tax losses carried forward for offset against future profits 338.6    
Supplementary carried forward tax deduction to be offset against future income € 87.2    
Number of subsidiaries operating on cost-plus basis | subsidiary 2    
Belgium [member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Amount of statutory tax losses carried forward for offset against future profits € 262.1    
Notional Interest Deduction [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Amounts that could give rise to deferred tax assets   2.6 2.6
Investment Deduction [Member]      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Amounts that could give rise to deferred tax assets 1.0 € 1.0 € 1.0
Expiry between 2018 and 2030      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Amount of unused tax losses carried forward 15.1    
Amount of statutory tax losses carried forward for offset against future profits € 16.8