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Financial instruments - Forward exchange contracts (Details) - Forward exchange contract
€ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2017
EUR (€)
Dec. 31, 2016
USD ($)
Dec. 31, 2016
EUR (€)
Disclosure of financial assets [line items]        
Hedge ineffectiveness $ 0      
Cash flow hedges        
Disclosure of financial assets [line items]        
Notional amount | €   € 109   € 92
Negative fair value $ 800,000      
Positive fair value     $ 300,000