Pension and Other Postretirement Benefits (Change in Fair Value of Level 3 Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Apr. 30, 2019	Apr. 30, 2018
Change in fair value of Level 3 Assets
Beginning balance	$ 780
Ending balance	754	$ 780
Level 3 [Member]
Change in fair value of Level 3 Assets
Beginning balance	4	4
Return on assets held at end of year		1
Sales and settlements	(1)	(1)
Ending balance	$ 3	$ 4