Pension and Other Postretirement Benefits (Change in Benefit Obligation) (Details) - USD ($) $ in Millions	12 Months Ended
	Apr. 30, 2021	Apr. 30, 2020	Apr. 30, 2019
Pension Benefits [Member]
Changes in present value of pension and other postretirement benefits
Obligation at beginning of year	$ 1,005	$ 908
Service cost	26	24	$ 24
Interest cost	25	31	34
Net actuarial loss (gain)	9	108
Retiree contributions	0	0
Benefits paid	(53)	(66)
Obligation at end of year	1,012	1,005	908
Medical and Life Insurance Benefits [Member]
Changes in present value of pension and other postretirement benefits
Obligation at beginning of year	51	50
Service cost	1	1	1
Interest cost	1	1	2
Net actuarial loss (gain)	(1)	2
Retiree contributions	1	1
Benefits paid	(4)	(4)
Obligation at end of year	$ 49	$ 51	$ 50