Long-term Debt Schedule of Long-term Debt Instruments (Details) - USD ($) $ in Millions	3 Months Ended
	Apr. 30, 2019	Apr. 30, 2018	Jan. 31, 2019
Long-term Debt, by Current and Noncurrent [Abstract]
Long-term debt, current maturities	$ 1,464	$ 1,576	$ 1,876
Long-term debt, excluding current maturities	47,425	29,477	43,520
Debt, long-term and short-term, combined amount	48,889		$ 45,396
Proceeds from issuance of long-term debt	3,978	$ 0
Repayments of long-term debt, current maturities	(364)
Repayments of long-term debt, excluding current maturities	0
Repayments of long-term debt, total	364
Long-term debt, current maturities, other Changes	(48)
Long-term debt, excluding current maturities, cther changes	(73)
Long-term debt, other changes	$ (121)