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Business Acquisitions - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended 12 Months Ended 5 Months Ended
Apr. 30, 2013
Jan. 31, 2013
Oct. 31, 2012
Jul. 31, 2012
Apr. 30, 2012
Jan. 31, 2012
Oct. 31, 2011
Jul. 31, 2011
Apr. 30, 2011
Jan. 31, 2011
Oct. 31, 2010
Jul. 31, 2010
Apr. 30, 2013
Property
Apr. 30, 2012
Property
Apr. 30, 2011
Apr. 30, 2013
Minimum [Member]
Y
Apr. 30, 2012
Minimum [Member]
Y
Apr. 30, 2013
Maximum [Member]
Y
Apr. 30, 2012
Maximum [Member]
Y
Apr. 30, 2013
Business Acquisition [Member]
Apr. 30, 2012
Business Acquisition [Member]
Apr. 30, 2013
Bestway Disposal Services And BBI Waste Services [Member]
Apr. 30, 2013
Bestway Disposal Services And BBI Waste Services [Member]
Class A Common Stock [Member]
Apr. 30, 2013
BBI Acquisition [Member]
Business Acquisition [Line Items]                                                
Total consideration $ 5,631       $ 2,217               $ 5,631 $ 2,217               $ 22,375    
Business acquisition, cash 5,080       2,102               5,080 2,102               19,725    
Business acquisition, hold backs to sellers                                       551 115      
Business acquisition, shares                                             625  
Business acquisition, stock value (2,650)                       (2,650)                 2,650    
Goodwill not expected to be deductible for tax purpose                                           12,916    
Number of business acquired                         6 5                    
Acquired intangible assets, amortization period                               5 5 10 10          
Deferred tax liability recorded through goodwill                                               5,084
Revenues $ 108,694 $ 112,167 $ 116,836 $ 117,638 $ 106,375 $ 111,520 $ 125,989 $ 124,066 $ 106,772 $ 109,076 $ 119,694 $ 119,143 $ 455,335 $ 467,950 $ 454,685                 $ 7,281