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Divestiture and Discontinued Operations (Narrative) (Detail) (USD $)
In Thousands, unless otherwise specified
 1 Months Ended 3 Months Ended 9 Months Ended
Aug. 31, 2012
Mar. 01, 2011
Jan. 31, 2013
Oct. 31, 2012
Jul. 31, 2012
Jan. 31, 2013
Jan. 31, 2012
Jan. 23, 2011
Facility
Divestitures [Line Items]                
Undiscounted purchase consideration $ 6,650              
Undiscounted purchase consideration paid in installments 21 years              
Loss on divestiture     353     353    
Number of material recovery facilities divested               17
Number of material recovery facilities retained               4
Commodities marketing agreement period               10 years
Percentage of purchaser to market commodities               100.00%
Remaining integrated MRFs               3
Gross proceeds from purchase and sale agreement   134,195            
Working capital and other purchase price adjustment   3,795            
Gain on disposal of discontinued operations, net of income taxes       $ 79 $ 646   $ 725