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Intangible Assets and Goodwill - Schedule of Activity and Balances Related to Goodwill (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Apr. 30, 2013
Apr. 30, 2012
Apr. 30, 2013
Eastern region [Member]
Apr. 30, 2012
Eastern region [Member]
Apr. 30, 2013
Western region [Member]
Apr. 30, 2012
Western region [Member]
Apr. 30, 2013
Recycling [Member]
Apr. 30, 2012
Recycling [Member]
Apr. 30, 2011
Recycling [Member]
Goodwill [Line Items]                  
Goodwill, beginning balance $ 101,706 $ 101,204 $ 58 $ 38 $ 89,458 $ 88,976 $ 12,190 $ 12,190 $ 12,190
Acquisitions 14,222 502 12,939 20 1,283 482      
Other     3,861   (3,861)        
Goodwill, ending balance $ 115,928 $ 101,706 $ 16,858 $ 58 $ 86,880 $ 89,458 $ 12,190 $ 12,190 $ 12,190