Short-term Borrowings and Long-term Debt (Schedule of Lines of Credit) (Details) - USD ($) $ in Millions	Jan. 31, 2018	Jan. 31, 2017
Line of Credit Facility [Line Items]
Available	$ 12,500	$ 12,500
Five Year Facility [Member]
Line of Credit Facility [Line Items]
Available	5,000	5,000
Three Hundred And Sixty Four Day Facility [Member]
Line of Credit Facility [Line Items]
Available	7,500	7,500
Drawn amount
Line of Credit Facility [Line Items]
Drawn	0	0
Drawn amount | Five Year Facility [Member]
Line of Credit Facility [Line Items]
Drawn	0	0
Drawn amount | Three Hundred And Sixty Four Day Facility [Member]
Line of Credit Facility [Line Items]
Drawn	0	0
Unused lines of Credit [Member]
Line of Credit Facility [Line Items]
Undrawn	12,500	12,500
Unused lines of Credit [Member] | Five Year Facility [Member]
Line of Credit Facility [Line Items]
Undrawn	5,000	5,000
Unused lines of Credit [Member] | Three Hundred And Sixty Four Day Facility [Member]
Line of Credit Facility [Line Items]
Undrawn	$ 7,500	$ 7,500