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Income taxes - Unrecognised deferred tax assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Unrecognised deferred tax assets [line items]    
Deductible temporary differences, basis $ 2,550 $ 2,439
Tax losses carried forward, basis 18,259 14,802
Total, unrecognised deferred tax assets, basis 20,809 17,241
Deductible temporary differences, unrecognised deferred tax asset 1,138 1,123
Tax losses carried forward, unrecognised deferred tax asset 4,366 3,940
Total unrecognised deferred tax assets $ 5,504 5,062
Percentage of unrecognised tax losses that can be carried forward indefinitely 11.00%  
Remaining unrecognised tax losses expiry date The majority of the remaining part of the unrecognised tax losses expire after 2030.  
United States [member]    
Unrecognised deferred tax assets [line items]    
Total unrecognised deferred tax assets $ 3,788 3,480
Angola [member]    
Unrecognised deferred tax assets [line items]    
Total unrecognised deferred tax assets 833 884
Ireland [member]    
Unrecognised deferred tax assets [line items]    
Total unrecognised deferred tax assets $ 191 $ 109