Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				6 Months Ended
	Oct. 31, 2019	Jul. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Oct. 31, 2019	Oct. 31, 2018
Earnings Per Share [Abstract]
Net income	$ 211.2	$ 154.6	$ 188.5	$ 133.0	$ 365.8	$ 321.5
Less: Net income allocated to participating securities	1.3		1.0		2.1	1.7
Net income allocated to common stockholders	$ 209.9		$ 187.5		$ 363.7	$ 319.8
Weighted-average common shares outstanding (in shares)	113.4		113.2		113.3	113.1
Add: Dilutive effect of stock options (in shares)	0.0		0.0		0.0	0.0
Weighted-average common shares outstanding - assuming dilution (in shares)	113.4		113.2		113.3	113.1
Net income per common share (in dollars per share)	$ 1.85		$ 1.66		$ 3.21	$ 2.83
Net Income - Assuming Dilution (in dollars per share)	$ 1.85		$ 1.66		$ 3.21	$ 2.83