Earnings Per Share (Details) (USD $) In Millions, except Share data	3 Months Ended
	Jul. 31, 2011	Jul. 31, 2010
Basic and diluted earnings per share
Basic and diluted net income	$ 118.1	$ 111.4
Income allocated to participating securities (restricted shares)		(0.1)
Net income available to common stockholders	$ 118.1	$ 111.3
Share data (in thousands):
Dilutive effect of stock options, SSARs, RSUs, and DSUs	1,039,000	815,000
Basic average common shares outstanding	144,828,000	146,570,000
Diluted average common shares outstanding	145,867,000	147,385,000
Basic earnings per share	$ 0.81	$ 0.76
Diluted earnings per share	$ 0.81	$ 0.76
Earnings Per Share (Textuals) [Abstract]
Stock options stock settled appreciation rights not dilutive	388,000	428,000