Pensions and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Jan. 31, 2021	Oct. 31, 2020	Jan. 31, 2020	Jan. 31, 2021	Jan. 31, 2020
Components of net periodic benefit cost
Settlement loss (gain)				$ 30.8	$ 0.0
Defined Benefit Pension Plans
Components of net periodic benefit cost
Service cost	$ 0.5		$ 0.4	1.4	1.2
Interest cost	3.3		5.2	11.1	15.7
Expected return on plan assets	(4.6)		(6.0)	(14.7)	(18.1)
Amortization of net actuarial loss (gain)	2.6		2.0	8.4	6.0
Amortization of prior service cost (credit)	0.2		0.2	0.6	0.6
Settlement loss (gain)	0.2		0.0	30.8	0.0
Net periodic benefit cost	2.2		1.8	37.6	5.4
Defined Benefit Pension Plans | Canada
Components of net periodic benefit cost
Settlement loss (gain)	0.0	$ 27.9
Decrease in pension obligations		$ (82.6)
Other Postretirement Benefits
Components of net periodic benefit cost
Service cost	0.5		0.4	1.4	1.3
Interest cost	0.4		0.5	1.3	1.7
Expected return on plan assets	0.0		0.0	0.0	0.0
Amortization of net actuarial loss (gain)	0.0		(0.1)	0.0	(0.2)
Amortization of prior service cost (credit)	(0.3)		(0.2)	(0.8)	(0.8)
Settlement loss (gain)	0.0		0.0	0.0	0.0
Net periodic benefit cost	$ 0.6		$ 0.6	$ 1.9	$ 2.0