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Shareholders' Equity (Schedule of Share-Based Compensation Expense) (Details) - USD ($)
$ in Millions
12 Months Ended
Jan. 31, 2018
Jan. 31, 2017
Jan. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense $ 626 $ 596 $ 448
Restricted stock and performance share units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense 234 237 134
Restricted stock units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense 368 332 292
Other      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense $ 24 $ 27 $ 22