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Related Party Transactions - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Apr. 30, 2013
Apr. 30, 2012
Apr. 30, 2011
Related Party Transaction [Line Items]      
Total services purchased $ 6,577 $ 2,612 $ 6,067
Outstanding accounts payable 1,189 45  
Leases monthly payments 25    
Capital lease expiration date Apr. 30, 2013    
Extension in lease Period 5 years    
Total expense charged to operations under lease agreement 286 300 311
Landfill post-closure cost 2,491 1,560  
Accrued costs associated with post-closure obligations 43,170 39,629 36,407
Recourse loan to an employee outstanding 147 144  
Landfills [Member]
      
Related Party Transaction [Line Items]      
Landfill owned 2    
Landfill post-closure cost 8 8 8
Accrued costs associated with post-closure obligations 100 84  
Recourse Debt [Member]
      
Related Party Transaction [Line Items]      
Recourse loan to an employee outstanding $ 147 $ 144  
Interest at Wall Street Journal Prime Rate 3.25%