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Asset retirement obligations - Summary of Present Value of Estimated Future Net Cash Outflows (Details)
Dec. 31, 2019
Dec. 31, 2018
Turkey    
Disclosure of Asset Retirement Obligations [line items]    
Inflation rate 1.80%  
Discount rate 1.90% 2.70%
Turkey | Bottom of range    
Disclosure of Asset Retirement Obligations [line items]    
Inflation rate   2.20%
Turkey | Top of range    
Disclosure of Asset Retirement Obligations [line items]    
Inflation rate   2.30%
Canada    
Disclosure of Asset Retirement Obligations [line items]    
Inflation rate 1.80%  
Discount rate 1.90% 2.70%
Canada | Bottom of range    
Disclosure of Asset Retirement Obligations [line items]    
Inflation rate   2.20%
Canada | Top of range    
Disclosure of Asset Retirement Obligations [line items]    
Inflation rate   2.30%
Greece | Bottom of range    
Disclosure of Asset Retirement Obligations [line items]    
Inflation rate 1.70% 2.20%
Discount rate 1.70% 2.50%
Greece | Top of range    
Disclosure of Asset Retirement Obligations [line items]    
Inflation rate 1.90% 2.30%
Discount rate 2.30% 2.90%
Romania    
Disclosure of Asset Retirement Obligations [line items]    
Inflation rate 1.90%  
Discount rate 2.30% 2.90%
Romania | Bottom of range    
Disclosure of Asset Retirement Obligations [line items]    
Inflation rate   2.20%
Romania | Top of range    
Disclosure of Asset Retirement Obligations [line items]    
Inflation rate   2.30%
Brazil    
Disclosure of Asset Retirement Obligations [line items]    
Inflation rate 1.60%  
Discount rate 1.60% 2.60%
Brazil | Bottom of range    
Disclosure of Asset Retirement Obligations [line items]    
Inflation rate   2.20%
Brazil | Top of range    
Disclosure of Asset Retirement Obligations [line items]    
Inflation rate   2.30%