Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended																12 Months Ended
	Apr. 30, 2020		Jan. 31, 2020		Oct. 31, 2019		Jul. 31, 2019		Apr. 30, 2019		Jan. 31, 2019		Oct. 31, 2018		Jul. 31, 2018		Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018
Earnings Per Share [Abstract]
Net income	$ 226.3		$ 187.4		$ 211.2		$ 154.6		$ 71.5		$ 121.4		$ 188.5		$ 133.0		$ 779.5	$ 514.4	$ 1,338.6
Net income allocated to participating securities																	4.4	2.6	6.8
Net income allocated to common stockholders																	$ 775.1	$ 511.8	$ 1,331.8
Weighted-average common shares outstanding (in shares)																	113.4	113.1	113.0
Dilutive effect of stock options (in shares)																	0.0	0.0	0.0
Weighted-average common shares outstanding - assuming dilution (in shares)																	113.4	113.1	113.0
Net income per common share (in dollars per share)	$ 1.98	[1]	$ 1.64	[1]	$ 1.85	[1]	$ 1.36	[1]	$ 0.63	[1]	$ 1.07	[1]	$ 1.66	[1]	$ 1.17	[1]	$ 6.84	$ 4.52	$ 11.79
Net income per common share - assuming dilution (in dollars per share)	$ 1.98	[1]	$ 1.64	[1]	$ 1.85	[1]	$ 1.36	[1]	$ 0.63	[1]	$ 1.07	[1]	$ 1.66	[1]	$ 1.17	[1]	$ 6.84	$ 4.52	$ 11.78

[1] Annual net income per common share may not equal the sum of the individual quarters due to differences in the average number of shares outstanding during the respective periods, primarily due to share repurchases.