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Post-employment benefits for associates - Additional information (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of defined benefit plans [line items]          
Past service cost, net defined benefit liability (asset)   $ 12      
Gains (losses) arising from settlements, net defined benefit liability (asset)   $ (12)      
Weighted average duration of defined benefit obligation     15 years 6 months 15 years 7 months 6 days  
Defined contribution plans charges     $ 136 $ 128 $ 105
Provisions and Other Non-current Liabilities [Member]          
Disclosure of defined benefit plans [line items]          
Gains (losses) arising from settlements, net defined benefit liability (asset) $ (164)        
Other Income [Member]          
Disclosure of defined benefit plans [line items]          
Past service cost, net defined benefit liability (asset) $ (164)        
Benefit obligation [member]          
Disclosure of defined benefit plans [line items]          
Top four countries, percentage     86.00%    
Pension plans [member] | UNITED KINGDOM | Foreign countries [member]          
Disclosure of defined benefit plans [line items]          
Maximum number of years to eliminate shortfall     10 years    
Other post-employment benefit plans [member] | Benefit obligation [member]          
Disclosure of defined benefit plans [line items]          
Associates covered, percentage     99.00%