Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Jan. 31, 2016	Jan. 31, 2015	Jan. 31, 2016	Jan. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 185.3	$ 160.9	$ 497.7	$ 435.2
Less: Net income allocated to participating securities	0.8	1.0	2.2	2.9
Net income allocated to common stockholders	$ 184.5	$ 159.9	$ 495.5	$ 432.3
Weighted-average common shares outstanding (in shares)	119,167,720	101,190,896	119,138,552	101,114,223
Add: Dilutive effect of stock options (in shares)	52,585	675	25,356	4,132
Weighted-average common shares outstanding - assuming dilution (in shares)	119,220,305	101,191,571	119,163,908	101,118,355
Net income per common share (in dollars per share)	$ 1.55	$ 1.58	$ 4.16	$ 4.28
Net Income - Assuming Dilution (in dollars per share)	$ 1.55	$ 1.58	$ 4.16	$ 4.28