Pensions and Other Postretirement Benefits (Details 4) - USD ($) $ in Millions	12 Months Ended
Apr. 30, 2020	Apr. 30, 2019
Defined Benefit Pension Plans [Member]
Accumulated other comprehensive income (loss)
Net actuarial gain (loss)	$ (199.7)	$ (157.2)
Prior service credit (cost)	(2.6)	(3.5)
Total recognized in accumulated other comprehensive income (loss)	$ (202.3)	$ (160.7)
Defined Benefit Pension Plans [Member] \| United States [Member]
Weighted-average assumptions used in determining benefit obligation
Discount rate	3.05%	3.99%
Rate of compensation increase	3.58%	3.56%
Defined Benefit Pension Plans [Member] \| CANADA
Weighted-average assumptions used in determining benefit obligation
Discount rate	2.95%	3.21%
Rate of compensation increase	3.00%	3.00%
Other Postretirement Benefits [Member]
Accumulated other comprehensive income (loss)
Net actuarial gain (loss)	$ 5.5	$ 10.2
Prior service credit (cost)	4.7	5.8
Total recognized in accumulated other comprehensive income (loss)	10.2	$ 16.0
One-percentage point annual change in the assumed health care cost trend rate
Effect on total service and interest cost components, increase	0.0
Effect on total service and interest cost components, decrease	0.0
Effect on benefit obligation, increase	1.1
Effect on benefit obligation, decrease	$ 1.1
Other Postretirement Benefits [Member] \| United States [Member]
Weighted-average assumptions used in determining benefit obligation
Discount rate	2.98%	3.91%
Rate of compensation increase	0.00%	0.00%
Other Postretirement Benefits [Member] \| CANADA
Weighted-average assumptions used in determining benefit obligation
Discount rate	2.93%	3.19%
Rate of compensation increase	0.00%	0.00%

Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

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Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

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Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate.

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Amount of increase in accumulated postretirement benefit obligation from one-percentage-point increase in assumed health care cost trend rate.

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