Pension and Other Postretirement Benefits (Funded Status Recorded on Accompanying Balance Sheets) (Details) - USD ($) $ in Millions	Apr. 30, 2017	Apr. 30, 2016
Funded status is recorded on the accompanying consolidated balance sheets
Accrued postretirement benefits	$ (314)	$ (353)
Pension Benefits [Member]
Funded status is recorded on the accompanying consolidated balance sheets
Accounts payable and accrued expenses	(5)	(4)
Accrued postretirement benefits	(265)	(300)
Net liability	(270)	(304)
Accumulated other comprehensive income (loss), before tax:
Net actuarial gain (loss)	(322)	(372)
Prior service credit (cost)	(4)	(4)
Total	(326)	(376)
Medical and Life Insurance Benefits [Member]
Funded status is recorded on the accompanying consolidated balance sheets
Accounts payable and accrued expenses	(3)	(3)
Accrued postretirement benefits	(49)	(53)
Net liability	(52)	(56)
Accumulated other comprehensive income (loss), before tax:
Net actuarial gain (loss)	(13)	(13)
Prior service credit (cost)	17	15
Total	$ 4	$ 2