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Stock-based compensation (Components of Stock-based Compensation Expense) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Components of stock-based compensation expense [Abstract]      
Total stock-based compensation expense, pretax $ 311 $ 329 $ 311
Tax benefit from stock-based compensation expense (67) (118) (112)
Total stock-based compensation expense, net of tax 244 211 199
RSUs [Member]      
Components of stock-based compensation expense [Abstract]      
Total stock-based compensation expense, pretax 165 174 177
Performance units [Member]      
Components of stock-based compensation expense [Abstract]      
Total stock-based compensation expense, pretax 117 133 123
Stock options [Member]      
Components of stock-based compensation expense [Abstract]      
Total stock-based compensation expense, pretax $ 29 $ 22 $ 11