Retirement Plans - Schedule of Pre-tax-mark-to-market Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	May 31, 2017	May 31, 2016	May 31, 2017	May 31, 2016	May 31, 2015
Actuarial Gain Loss By Component Pre Tax [Abstract]
Actual versus expected return on assets			$ (740)	$ 1,285	$ (35)
Discount rate changes			266	1,129	791
Demographic assumption experience			450	(916)	1,434
Total mark-to-market (gain) loss	$ (24)	$ 1,500	$ (24)	$ 1,498	$ 2,190