Schedule of Debt (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Oct. 31, 2017	Oct. 31, 2016	Oct. 31, 2017	Oct. 31, 2016	Jan. 31, 2017
Debt Instrument [Line Items]
Long-term debt due within one year	$ 3,257	$ 2,266	$ 3,257	$ 2,266	$ 2,256
Long-term debt	34,206	36,178	34,206	36,178	36,015
Total	37,463		37,463		$ 38,271
Proceeds from issuance of long-term debt			7,476	134
Repayments of long-term debt due within one year			(1,535)
Repayments of long-term debt			(7,324)
Repayments of long-term debt			8,859
Reclassifications of long-term debt			(2,500)
Reclassifications of long-term debt			2,500
Long-term debt, current maturities, other changes			36
Long-term debt, excluding current maturities, other changes			539
Long-term debt, other changes			575
Loss on extinguishment of debt	$ 1,344	$ 0	$ 2,132	$ 0