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Summary of Marketable Securities (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
6 Months Ended 6 Months Ended
Oct. 31, 2012
Apr. 30, 2012
Oct. 31, 2012
Investments Held in Trust
Oct. 31, 2011
Investments Held in Trust
Schedule Of Marketable Securities [Line Items]        
Obligations for which assets are held in trust $ 93.9 $ 82.6    
Increase (decrease) in fair value of trading securities     1.1 (4.1)
Company contribution 19.3      
Employee contribution 1.7      
Distributions 10.4      
Cash equivalents 74.1 60.5    
Restricted cash $ 10.1 $ 10.1