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Commitments and Contingencies - Additional Information (Detail) - USD ($)
9 Months Ended
Jan. 31, 2016
Apr. 30, 2015
May. 05, 2014
Schedule Of Commitments And Contingencies [Line Items]      
Environmental reserve in non-current liabilities $ 675,000 $ 675,000  
Amount placed in escrow     $ 3,000,000
Escrow deposit, remaining amount     $ 250,000
General and Administrative Expense      
Schedule Of Commitments And Contingencies [Line Items]      
Insurance recovery as a result of an insurance settlement agreement 1,800,000    
Minimum      
Schedule Of Commitments And Contingencies [Line Items]      
Compensatory damages sought 75,000    
Maximum      
Schedule Of Commitments And Contingencies [Line Items]      
Compensatory damages sought $ 1,500,000