Pension and Other Postretirement Benefits (Funded Status Recorded on Accompanying Balance Sheets) (Details) - USD ($) $ in Millions	Apr. 30, 2018	Apr. 30, 2017
Funded status is recorded on the accompanying consolidated balance sheets
Accrued postretirement benefits	$ (191)	$ (314)
Pension Benefits [Member]
Funded status is recorded on the accompanying consolidated balance sheets
Other assets	26	0
Accounts payable and accrued expenses	(5)	(5)
Accrued postretirement benefits	(144)	(265)
Net liability	(123)	(270)
Accumulated other comprehensive income (loss), before tax:
Net actuarial gain (loss)	(291)	(322)
Prior service credit (cost)	(9)	(4)
Total	(300)	(326)
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)	(49)
Net liability	(50)	(52)
Accumulated other comprehensive income (loss), before tax:
Net actuarial gain (loss)	(10)	(13)
Prior service credit (cost)	13	17
Total	$ 3	$ 4