Schedule of Debt (Details) - USD ($) $ in Millions	9 Months Ended
	Oct. 31, 2016	Oct. 31, 2015	Jan. 31, 2016
Debt Instrument [Line Items]
Long-term debt due within one year	$ 2,266	$ 2,746	$ 2,745
Long-term debt	36,178	38,617	38,214
Total	38,444		$ 40,959
Proceeds from issuance of long-term debt	134	41
Repayments of long-term debt	(2,040)	$ (4,422)
Reclassifications of long-term debt	(1,500)
Reclassifications of long-term debt	1,500
Long-term debt, current maturities, other changes	61
Long-term debt, excluding current maturities, other changes	(670)
Long-term debt, other changes	(609)
Unsecured debt
Debt Instrument [Line Items]
Repayments of long-term debt	$ (2,000)