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Derivative financial instruments - Carrying and notional amounts of hedges (Details)
number in Millions, $ in Millions
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Cash flow hedges    
Disclosure of detailed information about financial instruments [line items]    
Hedging instrument, assets $ 7 $ 35
Hedging instrument, liabilities (14) (35)
Fair value hedges    
Disclosure of detailed information about financial instruments [line items]    
Hedging instrument, assets 420 653
Hedging instrument, liabilities (1,234) $ (829)
Fair value hedges | Interest rate risk on finance debt    
Disclosure of detailed information about financial instruments [line items]    
Hedging instrument, assets 262  
Hedging instrument, liabilities (445)  
Nominal amounts of hedging instruments 24,513  
Fair value hedges | Interest rate and foreign currency risk on finance debt    
Disclosure of detailed information about financial instruments [line items]    
Hedging instrument, assets 158  
Hedging instrument, liabilities (789)  
Nominal amounts of hedging instruments 16,580  
Highly probable forecast capital expenditure | Cash flow hedges | Foreign exchange risk    
Disclosure of detailed information about financial instruments [line items]    
Hedging instrument, assets 5  
Hedging instrument, liabilities (14)  
Nominal amounts of hedging instruments 386  
Highly probable forecast sales | Cash flow hedges | Commodity price risk    
Disclosure of detailed information about financial instruments [line items]    
Hedging instrument, assets 2  
Hedging instrument, liabilities $ 0  
Nominal amounts of hedging instruments (in mmBtu) 145