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Summary of Marketable Securities (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
Jul. 31, 2013
Apr. 30, 2013
Schedule Of Marketable Securities [Line Items]    
Obligations for which assets are held in trust $ 115.4 $ 99.2
Cash equivalents 45.8 93.6
Restricted cash $ 0 $ 2.9