Pensions and Other Postretirement Benefits (Details 5) - USD ($) $ in Millions	12 Months Ended
	Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018
Change in plan assets:
Company contributions	$ 5.1	$ 29.3	$ 39.6
Pension Plan [Member]
Company's Canadian pension and other postretirement benefit plans
Benefit obligation at end of year	652.3	615.5	639.7
Fair value of plan assets at end of year	471.6	480.3	497.0
Funded status of the plans	(180.7)	(135.2)
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 net actuarial loss (gain)	7.9	8.3	11.5
Termination benefit cost	0.2	0.0	0.0
Net periodic benefit cost (credit)	7.5	15.1	12.7
Change in plan assets:
Company contributions	5.1	29.3
Benefits paid	(39.6)	(33.9)
Actual return on plan assets	34.3	19.6
Foreign currency translation adjustments	(3.6)	(4.0)
Other Postretirement Benefits Plan [Member]
Company's Canadian pension and other postretirement benefit plans
Benefit obligation at end of year	74.5	70.1	65.9
Fair value of plan assets at end of year	0.0	0.0	0.0
Funded status of the plans	(74.5)	(70.1)
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 net actuarial loss (gain)	(0.3)	(0.6)	(0.3)
Termination benefit cost	0.0	0.2	0.0
Net periodic benefit cost (credit)	2.7	2.5	$ 2.4
Change in plan assets:
Company contributions	3.8	4.7
Benefits paid	(3.8)	(4.7)
Actual return on plan assets	0.0	0.0
Foreign currency translation adjustments	0.0	0.0
CANADA | Pension Plan [Member]
Company's Canadian pension and other postretirement benefit plans
Benefit obligation at end of year	80.1	84.8
Fair value of plan assets at end of year	91.0	92.1
Funded status of the plans	10.9	7.3
Components of net periodic benefit cost:
Service cost	0.1	0.1
Interest cost	2.3	2.7
Expected return on plan assets	(4.4)	(4.8)
Amortization of net actuarial loss (gain)	1.1	0.9
Termination benefit cost	0.2	0.0
Net periodic benefit cost (credit)	(0.7)	(1.1)
Change in plan assets:
Company contributions	0.1	0.1
Benefits paid	(6.8)	(6.5)
Actual return on plan assets	9.2	6.1
Foreign currency translation adjustments	(3.6)	(3.9)
CANADA | Other Postretirement Benefits Plan [Member]
Company's Canadian pension and other postretirement benefit plans
Benefit obligation at end of year	6.8	7.1
Fair value of plan assets at end of year	0.0	0.0
Funded status of the plans	(6.8)	(7.1)
Components of net periodic benefit cost:
Service cost	0.0	0.0
Interest cost	0.2	0.2
Expected return on plan assets	0.0	0.0
Amortization of net actuarial loss (gain)	0.0	0.0
Termination benefit cost	0.0	0.0
Net periodic benefit cost (credit)	0.2	0.2
Change in plan assets:
Company contributions	0.3	0.5
Benefits paid	(0.3)	(0.5)
Actual return on plan assets	0.0	0.0
Foreign currency translation adjustments	$ 0.0	$ 0.0