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Stock-based compensation (Components of Stock-based Compensation Expense) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Components of stock-based compensation expense [Abstract]      
Total stock-based compensation expense, pretax $ 330 $ 308 $ 311
Tax benefit from stock-based compensation expense (72) (67) (67)
Total stock-based compensation expense, net of tax 258 241 244
RSUs [Member]      
Components of stock-based compensation expense [Abstract]      
Total stock-based compensation expense, pretax 178 168 165
Performance units [Member]      
Components of stock-based compensation expense [Abstract]      
Total stock-based compensation expense, pretax 118 105 117
Stock options [Member]      
Components of stock-based compensation expense [Abstract]      
Total stock-based compensation expense, pretax $ 34 $ 35 $ 29