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Income taxes - Deferred tax balances (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Deferred tax assets and liabilities        
NET DEFERRED TAX LIABILITY $ (3,310) $ (5,642) $ (4,827) $ (5,622)
Net operating losses and tax carry forwards        
Deferred tax assets and liabilities        
Deferred tax asset 5,106 3,752 3,779  
Employee benefits        
Deferred tax assets and liabilities        
Deferred tax asset 1,004 970 995  
Other temporary non-deductible provisions or tax deductions        
Deferred tax assets and liabilities        
Deferred tax asset 9,068 8,660 8,409  
Deferred tax liability (3,847) (2,995) (2,541)  
Differences in depreciations        
Deferred tax assets and liabilities        
Deferred tax liability $ (14,641) $ (16,029) $ (15,469)