Pensions and Other Postretirement Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018
Defined Benefit Pension Plans [Member]
Components of net periodic benefit cost:
Service cost	$ 1.6	$ 2.1	$ 5.2
Interest cost	20.9	23.2	21.6
Expected return on plan assets	(24.1)	(26.8)	(28.8)
Amortization of prior service cost (credit)	0.9	0.9	0.9
Amortization of net actuarial loss (gain)	7.9	8.3	11.5
Curtailment loss (gain)	0.0	0.3	0.0
Settlement loss (gain)	0.1	7.1	2.3
Termination benefit cost	0.2	0.0	0.0
Net periodic benefit cost	7.5	15.1	12.7
Other Postretirement Benefits [Member]
Components of net periodic benefit cost:
Service cost	1.8	1.9	2.0
Interest cost	2.3	2.3	2.1
Expected return on plan assets	0.0	0.0	0.0
Amortization of prior service cost (credit)	(1.1)	(1.3)	(1.4)
Amortization of net actuarial loss (gain)	(0.3)	(0.6)	(0.3)
Curtailment loss (gain)	0.0	0.0	0.0
Settlement loss (gain)	0.0	0.0	0.0
Termination benefit cost	0.0	0.2	0.0
Net periodic benefit cost	$ 2.7	$ 2.5	$ 2.4