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Divestiture Transactions and Discontinued Operations - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Jul. 31, 2013
Jul. 31, 2012
Apr. 30, 2014
Apr. 30, 2012
Divestitures [Line Items]        
Undiscounted purchase consideration paid in installments 5 years 21 years    
Loss on disposal of discontinued operations, net of tax $ (378)   $ (378) $ 725
BioFuels Assets [Member]
        
Divestitures [Line Items]        
Undiscounted purchase consideration 2,000      
Maine Energy Facility [Member]
        
Divestitures [Line Items]        
Undiscounted purchase consideration   $ 6,650