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Retirement Plans - Schedule of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Feb. 28, 2017
Feb. 29, 2016
Feb. 28, 2017
Feb. 29, 2016
Pension Plans [Member]        
Net Periodic Benefit Cost        
Service cost $ 180 $ 166 $ 540 $ 497
Interest cost 293 295 879 885
Expected return on plan assets (384) (377) (1,156) (1,131)
Amortization of prior service credit and other (32) (31) (90) (91)
Total net periodic benefit cost 57 53 173 160
Postretirement Healthcare Plans [Member]        
Net Periodic Benefit Cost        
Service cost 9 10 27 30
Interest cost 10 10 30 31
Total net periodic benefit cost $ 19 $ 20 $ 57 $ 61