Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 26.8	$ 20.7	$ 15.4
Related income tax benefit	6.4	4.9	4.6
Selling, Distribution And Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	26.4	20.1	13.7
Other special project costs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 0.4	$ 0.6	$ 1.7