Pensions and Other Postretirement Benefits - Narrative (Details) $ in Millions	12 Months Ended
	Apr. 30, 2021 USD ($)	Apr. 30, 2020 USD ($) plan	Apr. 30, 2019 USD ($)	Dec. 31, 2019
Pensions and Other Postretirement Benefits (Additional Textual) [Abstract]
Eligibility condition for covered employees to avail the benefits of unfunded, defined postretirement plans that provide health care and life insurance benefits		when they reach age 55 and have attained 10 years of credited service
Expected amount to be recognized during 2021 of amortization of net actuarial losses in net periodic benefit cost		$ (11.7)
Expected amount to be recognized during 2021 in prior service credit in net periodic benefit cost		(0.1)
Expected benefit payments for the defined benefit pension and other postretirement in 2021		43.7
Expected benefit payments for the defined benefit pension and other postretirement in 2022		45.9
Expected benefit payments for the defined benefit pension and other postretirement in 2023		45.6
Expected benefit payments for the defined benefit pension and other postretirement in 2024		45.2
Expected benefit payments for the defined benefit pension and other postretirement in 2025		45.4
Expected benefit payments for the defined benefit pension and other postretirement in 2026 through 2030		$ 213.3
Number of multiemployer pension plans | plan		1
Multiemployer plan contributions		$ 2.2	$ 2.3
Multiemployer plan actual funded status				50.40%
Forecast [Member]
Pensions and Other Postretirement Benefits (Additional Textual) [Abstract]
Multiemployer plan contributions	$ 2.5
Fixed - Income Securities [Member]
Pensions and Other Postretirement Benefits (Additional Textual) [Abstract]
Approximate percentage of assets to be invested in company's current investment policy		65.00%
Other Postretirement Benefits Plan [Member] | United States [Member]
Pensions and Other Postretirement Benefits (Additional Textual) [Abstract]
Assumed health care trend rate for next fiscal year		6.30%
Assumed health care trend rate for participants under age 65		5.00%
Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate		2026
Other Postretirement Benefits Plan [Member] | CANADA
Pensions and Other Postretirement Benefits (Additional Textual) [Abstract]
Assumed health care trend rate for next fiscal year		4.50%
Assumed health care trend rate for participants under age 65		4.50%
Defined Benefit Pension Plans [Member]
Pensions and Other Postretirement Benefits (Additional Textual) [Abstract]
Defined benefit plan actual rate of return on plan assets		7.80%	3.80%
Expected amount to be contributed by the Company to the defined benefit pension plans in 2021		$ 1.0
Expected direct benefit payments to be made by the Company in 2021		$ 13.5
Maximum
Pensions and Other Postretirement Benefits (Additional Textual) [Abstract]
High Quality Corporate Bonds with Duration Range		13 years
Maximum
Pensions and Other Postretirement Benefits (Additional Textual) [Abstract]
High Quality Corporate Bonds with Duration Range		10 years