Income Taxes - Schedule of Significant Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	May 31, 2017	May 31, 2016
Components Of Deferred Tax Assets And Liabilities [Abstract]
Property, equipment, leases and intangibles	$ 124	$ 129
Employee benefits	1,951	2,453
Self-insurance accruals	745	681
Other	692	528
Net operating loss/credit carryforwards	1,069	925
Valuation allowances	(738)	(738)
Deferred Tax Assets, Net	3,843	3,978
Property, equipment, leases and intangibles	4,993	4,767
Other	660	343
Deferred Tax Liabilities	$ 5,653	$ 5,110