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Restricted Cash / Restricted Assets - Summary of Restricted Cash or Restricted Asset (Detail) (USD $)
In Thousands, unless otherwise specified
Apr. 30, 2013
Apr. 30, 2012
Current:    
Restricted cash / restricted assets $ 76 $ 76
Non Current:    
Restricted cash / restricted assets 545 424
Landfills [Member]
   
Current:    
Restricted cash / restricted assets 76 76
Non Current:    
Restricted cash / restricted assets $ 545 $ 424