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Share-based compensation (Pre-tax and After-tax Share-based Compensation Expense) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Restricted stock      
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Share-based compensation expense, pre-tax $ 259 $ 268 $ 143
Share-based compensation expense, after-tax 151 167 84
Stock options      
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Share-based compensation expense, pre-tax 41 33 31
Share-based compensation expense, after-tax [1] $ 26 20 21
Additional Paid-in Capital      
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Tax benefit on share-based compensation expense   $ 32 $ 26
[1] (1) Excludes windfall tax benefit for share-based compensation recognized as a direct adjustment to Additional paid-in capital of $32 million and $26 million for the years ended December 31, 2016 and 2015, respectively.