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Income taxes - Unrecognised deferred tax assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Unrecognised deferred tax assets [line items]    
Deductible temporary differences, basis $ 2,866 $ 2,550
Unused tax credits, basis 0 0
Tax losses carried forward, basis 23,434 18,259
Total unrecognised deferred tax assets, basis 26,300 20,809
Deductible temporary differences, unrecognised deferred tax asset 1,204 1,138
Unused tax credits 212 0
Tax losses carried forward, unrecognised deferred tax asset 5,677 4,366
Total unrecognised deferred tax assets $ 7,093 5,504
Percentage of unrecognised tax losses that can be carried forward indefinitely 14.00%  
Remaining unrecognised tax losses expiry date The majority of the remaining part of the unrecognised tax losses expire after 2031.  
United States [member]    
Unrecognised deferred tax assets [line items]    
Total unrecognised deferred tax assets $ 4,649 3,788
Angola [member]    
Unrecognised deferred tax assets [line items]    
Total unrecognised deferred tax assets $ 740 $ 833