Pensions and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Oct. 31, 2019	Oct. 31, 2018	Oct. 31, 2019	Oct. 31, 2018
Defined Benefit Pension Plans
Components of net periodic benefit cost
Service cost	$ 0.4	$ 0.5	$ 0.8	$ 1.1
Interest cost	5.3	5.8	10.5	11.7
Expected return on plan assets	(6.1)	(6.7)	(12.1)	(13.5)
Amortization of net actuarial loss (gain)	2.0	2.1	4.0	4.1
Amortization of prior service cost (credit)	0.2	0.3	0.4	0.5
Net periodic benefit cost	1.8	2.0	3.6	3.9
Other Postretirement Benefits
Components of net periodic benefit cost
Service cost	0.4	0.5	0.9	0.9
Interest cost	0.6	0.6	1.2	1.2
Expected return on plan assets	0.0	0.0	0.0	0.0
Amortization of net actuarial loss (gain)	0.0	(0.2)	(0.1)	(0.3)
Amortization of prior service cost (credit)	(0.3)	(0.3)	(0.6)	(0.6)
Net periodic benefit cost	$ 0.7	$ 0.6	$ 1.4	$ 1.2