Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Oct. 31, 2018	Oct. 31, 2017	Oct. 31, 2018	Oct. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 188.5	$ 194.6	$ 321.5	$ 321.4
Less: Net income allocated to participating securities	1.0	1.1	1.7	1.7
Net income allocated to common stockholders	$ 187.5	$ 193.5	$ 319.8	$ 319.7
Weighted-average common shares outstanding (in shares)	113.2	113.0	113.1	113.0
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)	113.2	113.0	113.1	113.0
Net income per common share (in dollars per share)	$ 1.66	$ 1.71	$ 2.83	$ 2.83
Net Income - Assuming Dilution (in dollars per share)	$ 1.66	$ 1.71	$ 2.83	$ 2.83