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Financial risk management - D. Financial risk management (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Financial Instruments [Abstract]    
Target percent debt in local currency 40.00%  
Target percent of debt fixed rate 75.00%  
Target percent of debt floating rate 25.00%  
Disclosure of detailed information about hedging instruments [line items]    
Derivative financial liabilities $ 12 $ (17)
Cash flow hedges    
Disclosure of detailed information about hedging instruments [line items]    
Derivative financial assets $ 12  
Derivative financial liabilities   $ (17)