Schedule of Debt (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jul. 31, 2017	Jul. 31, 2016	Jul. 31, 2017	Jul. 31, 2016	Jan. 31, 2017
Debt Instrument [Line Items]
Loss on Extinguishment of Debt	$ 788	$ 0	$ 788	$ 0
Long-term debt due within one year	3,254	2,265	3,254	2,265	$ 2,256
Long-term debt	33,706	$ 36,673	33,706	36,673	36,015
Total	$ 36,960		36,960		$ 38,271
Proceeds from issuance of long-term debt			1,503	$ 130
Repayments of long-term debt due within one year			(1,526)
Repayments of long-term debt			(1,875)
Repayments of long-term debt			3,401
Reclassifications of long-term debt			(2,500)
Reclassifications of long-term debt			2,500
Long-term debt, current maturities, other changes			24
Long-term debt, excluding current maturities, other changes			563
Long-term debt, other changes			587
Repayment of Long-term debt, Premium Paid on Debt Tender			$ 800