Retirement Plans (Details 10) - USD ($) $ in Millions	12 Months Ended
	May. 31, 2015	May. 31, 2014	May. 31, 2013
Pension Plans [Member]
Net Periodic Benefit Cost
Service cost	$ 653	$ 657	$ 692
Interest cost	1,096	1,055	968
Expected return on plan assets	(1,678)	(1,495)	(1,383)
Amortization of prior service credit	(115)	(115)	(114)
Recognized actuarial losses (gains) and other	2,190	7	(1,350)
Total net periodic benefit cost	2,146	109	(1,187)
Postretirement Healthcare Plans [Member]
Net Periodic Benefit Cost
Service cost	40	38	42
Interest cost	41	40	36
Recognized actuarial losses (gains) and other	6	5	(17)
Total net periodic benefit cost	$ 87	$ 83	$ 61