Pensions and Other Postretirement Benefits (Details 7) - USD ($) $ in Millions		Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Asset Value Of Plan Assets	[1]	$ 5.1	$ 10.1
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value		471.6	480.3	$ 497.0
Defined Benefit Pension Plans [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value		466.5	470.2
Defined Benefit Pension Plans [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value		424.7	412.9
Defined Benefit Pension Plans [Member] | Significant Observable Inputs (Level 2) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value		41.8	57.3
Defined Benefit Pension Plans [Member] | Significant Unobservable Inputs (Level 3) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value		0.0	0.0
Defined Benefit Pension Plans [Member] | Cash and cash equivalents [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[2]	2.1	0.5
Defined Benefit Pension Plans [Member] | Cash and cash equivalents [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[2]	2.1	0.5
Defined Benefit Pension Plans [Member] | Cash and cash equivalents [Member] | Significant Observable Inputs (Level 2) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[2]	0.0	0.0
Defined Benefit Pension Plans [Member] | Cash and cash equivalents [Member] | Significant Unobservable Inputs (Level 3) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[2]	0.0	0.0
Defined Benefit Pension Plans [Member] | U.S. Equity Securities [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[3]	51.2	67.5
Defined Benefit Pension Plans [Member] | U.S. Equity Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[3]	51.2	65.7
Defined Benefit Pension Plans [Member] | U.S. Equity Securities [Member] | Significant Observable Inputs (Level 2) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[3]	0.0	1.8
Defined Benefit Pension Plans [Member] | U.S. Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[3]	0.0	0.0
Defined Benefit Pension Plans [Member] | International Equity Securities [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[4]	65.9	83.5
Defined Benefit Pension Plans [Member] | International Equity Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[4]	65.9	74.3
Defined Benefit Pension Plans [Member] | International Equity Securities [Member] | Significant Observable Inputs (Level 2) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[4]	0.0	9.2
Defined Benefit Pension Plans [Member] | International Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[4]	0.0	0.0
Defined Benefit Pension Plans [Member] | Bonds [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[5]	212.1	220.6
Defined Benefit Pension Plans [Member] | Bonds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[5]	212.1	220.6
Defined Benefit Pension Plans [Member] | Bonds [Member] | Significant Observable Inputs (Level 2) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[5]	0.0	0.0
Defined Benefit Pension Plans [Member] | Bonds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[5]	0.0	0.0
Defined Benefit Pension Plans [Member] | Fixed income [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[6]	93.4	51.8
Defined Benefit Pension Plans [Member] | Fixed income [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[6]	93.4	51.8
Defined Benefit Pension Plans [Member] | Fixed income [Member] | Significant Observable Inputs (Level 2) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[6]	0.0	0.0
Defined Benefit Pension Plans [Member] | Fixed income [Member] | Significant Unobservable Inputs (Level 3) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[6]	0.0	0.0
Defined Benefit Pension Plans [Member] | Other Investments [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[7]	41.8	46.3
Defined Benefit Pension Plans [Member] | Other Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[7]	0.0	0.0
Defined Benefit Pension Plans [Member] | Other Investments [Member] | Significant Observable Inputs (Level 2) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[7]	41.8	46.3
Defined Benefit Pension Plans [Member] | Other Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Total financial assets measured at fair value	[7]	$ 0.0	$ 0.0

[1] This category is comprised of a private equity fund that consists primarily of limited partnership interests in corporate finance and venture capital funds, as well as a private limited investment partnership. The fair value estimates of the private equity fund and private limited investment partnership are based on the underlying funds’ net asset values. Furthermore, as a practical expedient equivalent to our defined benefit plan’s ownership interest in the partners’ capital, a proportionate share of the net assets is attributed and further corroborated by our review. The private equity fund and private limited investment partnership are non-redeemable, and the return of principal is based on the liquidation of the underlying assets. In accordance with ASU 2015-07, the private equity fund and private limited investment partnership are removed from the total financial assets measured at fair value and disclosed separately.

[2] This category includes money market holdings with maturities of three months or less and are classified as Level 1 assets. Based on the short-term nature of these assets, carrying value approximates fair value.

[3] This category is invested in a diversified portfolio of common stocks and index funds that primarily invest in U.S. stocks with broad market capitalization ranges similar to those found in the S&P 500 Index and/or the various Russell Indices and are traded on active exchanges. The Level 1 assets are valued using quoted market prices for identical securities in active markets. The Level 2 asset in 2019 was comprised of a pooled fund that consists of equity securities traded on active exchanges.

[4] This category is invested primarily in common stocks and other equity securities traded on active exchanges of foreign issuers located outside the U.S. The fund invests primarily in developed countries, but may also invest in emerging markets. The Level 1 assets are valued using quoted market prices for identical securities in active markets. The Level 2 asset in 2019 was comprised of a pooled fund that consists of equity securities traded on active exchanges.

[5] This category is primarily comprised of bond funds, which seek to duplicate the return characteristics of high-quality U.S. and foreign corporate bonds with a duration range of 10 to 13 years, as well as various U.S. Treasury Separate Trading of Registered Interest and Principal holdings, with wide-ranging maturity dates. These assets are valued using quoted market prices for identical securities in active markets and are classified as Level 1 assets.

[6] This category is comprised of fixed-income funds that invest primarily in government-related bonds of non-U.S. issuers and include investments in the Canadian, as well as emerging markets. These assets are valued using quoted market prices for identical securities in active markets and are classified as Level 1 assets.

[7] This category is comprised of a real estate fund whereby the underlying investments are contained in the Canadian market, and a common collective trust fund investing in direct commercial property funds. The real estate fund and the collective trust fund investing in direct commercial property are classified as Level 2 assets, whereby the underlying securities are valued utilizing quoted market prices for identical securities in active markets and based on the quoted market prices of the underlying investments in the common collective trust, respectively.