Schedule of Debt (Details) - USD ($) $ in Millions	3 Months Ended
	Apr. 30, 2016	Apr. 30, 2015	Jan. 31, 2016
Debt Instrument [Line Items]
Long-term debt due within one year	$ 2,257	$ 6,041	$ 2,745
Long-term debt	37,151	38,867	38,214
Total	39,408		$ 40,959
Proceeds from issuance of long-term debt	127	43
Repayments of long-term debt	(2,013)	$ (915)
Reclassifications of long-term debt	(1,500)
Reclassifications of long-term debt	1,500
Long-term debt, current maturities, other changes	25
Long-term debt, excluding current maturities, other changes	310
Long-term debt, other changes	335
Unsecured debt
Debt Instrument [Line Items]
Repayments of long-term debt	$ (2,000)