Schedule of Debt (Details) - USD ($) $ in Millions	3 Months Ended
	Apr. 30, 2017	Apr. 30, 2016	Jan. 31, 2017
Debt Instrument [Line Items]
Long-term debt due within one year	$ 3,256	$ 2,257	$ 2,256
Long-term debt	33,774	37,151	36,015
Total	37,030		$ 38,271
Proceeds from issuance of long-term debt	0	127
Repayments of long-term debt	(1,513)	$ (2,013)
Reclassifications of long-term debt	(2,500)
Reclassifications of long-term debt	2,500
Long-term debt, current maturities, other changes	13
Long-term debt, excluding current maturities, other changes	259
Long-term debt, other changes	272
Unsecured debt
Debt Instrument [Line Items]
Repayments of long-term debt	$ (1,500)