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Goodwill and Intangible Assets - Schedule of Activity and Balances Related to Goodwill (Detail) (USD $)
In Thousands, unless otherwise specified
 9 Months Ended
Jan. 31, 2014
Goodwill And Intangible Assets [Line Items]  
Goodwill, beginning balance $ 115,928
Acquisitions 3,136
Goodwill, ending balance 119,064
Eastern Region [Member]
  
Goodwill And Intangible Assets [Line Items]  
Goodwill, beginning balance 16,858
Acquisitions 558
Goodwill, ending balance 17,416
Western Region [Member]
  
Goodwill And Intangible Assets [Line Items]  
Goodwill, beginning balance 86,880
Acquisitions 786
Goodwill, ending balance 87,666
Recycling [Member]
  
Goodwill And Intangible Assets [Line Items]  
Goodwill, beginning balance 12,190
Acquisitions 63
Goodwill, ending balance 12,253
Other [Member]
  
Goodwill And Intangible Assets [Line Items]  
Goodwill, beginning balance   
Acquisitions 1,729
Goodwill, ending balance $ 1,729