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Retirement Plans - Summary of Retirement Plan Costs (Details) - USD ($)
$ in Millions
12 Months Ended
May 31, 2017
May 31, 2016
May 31, 2015
Pension And Other Postretirement Benefit Expense [Abstract]      
Defined benefit pension plans $ 234 $ 214 $ (41)
Defined contribution plans 480 416 385
Postretirement healthcare plans 76 82 81
Retirement plans mark-to-market adjustment (24) 1,498 2,190
Retirement plans costs $ 766 $ 2,210 $ 2,615