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Divestiture Transactions and Discontinued Operations - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 1 Months Ended 3 Months Ended 12 Months Ended 3 Months Ended 12 Months Ended 3 Months Ended
Mar. 01, 2011
Apr. 30, 2013
Jan. 31, 2013
Jul. 31, 2012
Oct. 31, 2011
Jul. 31, 2011
Apr. 30, 2011
Apr. 30, 2012
Apr. 30, 2011
Jan. 31, 2011
Facility
Jan. 31, 2011
Trilogy Glass [Member]
Apr. 30, 2011
Trilogy Glass [Member]
Jul. 31, 2011
Southeastern Massachusetts [Member]
Divestitures [Line Items]                          
Gain on disposal of discontinued operations   $ 3,261                      
Gross proceeds from purchase and sale agreement 134,195     6,650           130,400     7,750
Number of material recovery facilities divested                   17      
Number of MRFs retained                   4      
Commodities marketing agreement period                   10 years      
Percentage of Purchaser to market commodities                   100.00%      
Remaining integrated MRFs                   3      
Working capital and other purchase price adjustment 3,795                        
Proceeds from sale of assets 122,953                   1,840   7,533
Gain (loss) on disposal of discontinued operations, net of tax         79 646 43,718 725 43,590     128  
Undiscounted purchase consideration paid in installments       21 years                  
Loss on divestiture     (353)                    
Reversal of initial loss on divestiture   353                      
Gain on sale of assets                         $ 3,502