Pensions and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Oct. 31, 2018	Oct. 31, 2017	Oct. 31, 2018	Oct. 31, 2017
Defined Benefit Pension Plans [Member]
Components of net periodic benefit cost
Service cost	$ 0.5	$ 1.7	$ 1.1	$ 3.4
Interest cost	5.8	5.4	11.7	10.8
Expected return on plan assets	(6.7)	(7.2)	(13.5)	(14.4)
Amortization of net actuarial loss (gain)	2.1	2.8	4.1	5.7
Amortization of prior service cost (credit)	0.3	0.3	0.5	0.5
Net periodic benefit cost	2.0	3.0	3.9	6.0
Other Postretirement Benefits [Member]
Components of net periodic benefit cost
Service cost	0.5	0.5	0.9	1.0
Interest cost	0.6	0.6	1.2	1.1
Expected return on plan assets	0.0	0.0	0.0	0.0
Amortization of net actuarial loss (gain)	(0.2)	(0.1)	(0.3)	(0.2)
Amortization of prior service cost (credit)	(0.3)	(0.4)	(0.6)	(0.7)
Net periodic benefit cost	$ 0.6	$ 0.6	$ 1.2	$ 1.2