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Derivative Financial Instruments (Details 3) - USD ($)
$ in Millions
12 Months Ended
Apr. 30, 2015
Apr. 30, 2014
Apr. 30, 2013
Unallocated Derivative Gains (Losses)      
Net (losses) gains on mark-to-market valuation of unallocated derivative positions $ (39.7) $ 8.5  
Net losses (gains) on derivative positions reclassified to segment operating profit 15.2 (3.2)  
Net mark-to-market valuation of certain derivative positions recognized in unallocated derivative (losses) gains $ (24.5) $ 5.3 $ 6.6