Taxes (Schedule of Deferred Tax Balances) (Details) - USD ($) $ in Millions	Jan. 31, 2018	Jan. 31, 2017
Income Tax Disclosure [Abstract]
Loss and tax credit carryforwards	$ 1,989	$ 3,633
Accrued liabilities	2,482	3,437
Share-based compensation	217	309
Other	1,251	1,474
Total deferred tax assets	5,939	8,853
Valuation allowances	(1,843)	(1,494)
Deferred tax assets, net of valuation allowance	4,096	7,359
Property and equipment	3,954	6,435
Inventories	1,153	1,808
Other	941	1,884
Total deferred tax liabilities	6,048	10,127
Net deferred tax liabilities	$ 1,952	$ 2,768