Schedule of Debt (Details) - USD ($) $ in Millions	3 Months Ended
	Apr. 30, 2018	Jan. 31, 2018	Apr. 30, 2017
Debt Instrument [Line Items]
Long-term debt due within one year	$ 1,576	$ 3,738	$ 3,256
Long-term debt	29,477	30,045	$ 33,774
Total	31,053	$ 33,783
Repayments of long-term debt due within one year	(2,521)
Repayments of long-term debt	0
Repayments of long-term debt	2,521
Reclassifications of long-term debt	(364)
Reclassifications of long-term debt	364
Amounts reclassified to short-term debt, total	0
Long-term debt, current maturities, other changes	(5)
Long-term debt, excluding current maturities, other changes	(204)
Long-term debt, other changes	$ (209)