Pension and Other Postretirement Benefits (Funded Status Recorded on Accompanying Balance Sheets) (Details) - USD ($) $ in Millions	Apr. 30, 2019	Apr. 30, 2018
Funded status is recorded on the accompanying consolidated balance sheets
Accrued postretirement benefits	$ (197)	$ (191)
Pension Benefits [Member]
Funded status is recorded on the accompanying consolidated balance sheets
Other assets	2	26
Accounts payable and accrued expenses	(6)	(5)
Accrued postretirement benefits	(150)	(144)
Net liability	(154)	(123)
Accumulated other comprehensive income (loss), before tax:
Net actuarial gain (loss)	(298)	(291)
Prior service credit (cost)	(8)	(9)
Total	(306)	(300)
Medical and Life Insurance Benefits [Member]
Funded status is recorded on the accompanying consolidated balance sheets
Other assets	0	0
Accounts payable and accrued expenses	(3)	(3)
Accrued postretirement benefits	(47)	(47)
Net liability	(50)	(50)
Accumulated other comprehensive income (loss), before tax:
Net actuarial gain (loss)	(10)	(10)
Prior service credit (cost)	10	13
Total	$ 0	$ 3