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Commitments and Contingencies - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 1 Months Ended 3 Months Ended 12 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended
Apr. 30, 2011
May 31, 2009
Apr. 30, 2011
Apr. 30, 2010
Apr. 30, 2009
Jan. 31, 2009
Apr. 30, 2013
Single_Stream_Material_Recovery_Facilities
Executives
Apr. 30, 2012
Apr. 30, 2011
Nov. 13, 2012
D
May 31, 2011
Jan. 18, 2011
Jan. 30, 2009
Apr. 30, 2006
Agreement
Apr. 30, 2004
Agreement
Dec. 20, 2000
acre
Apr. 30, 2013
Maximum [Member]
Apr. 30, 2013
Minimum [Member]
Sep. 30, 2011
Waste Stream Inc [Member]
Nov. 30, 2011
Motors Liquidation Trust [Member]
Apr. 30, 2013
Financing lease obligations [Member]
Apr. 30, 2012
Financing lease obligations [Member]
Apr. 30, 2011
Financing lease obligations [Member]
Contingencies And Commitments [Line Items]                                              
Term of financial lease obligation                                         7 years    
Financial lease obligation, fixed rate of interest             6.70%                                
Capitalized assets                                         $ 3,213 $ 3,213  
Depreciation expense on property, plant and equipment             34,065 37,829 35,939                       293 293 293
Number of single stream MRFs             2                                
Operating facilities and equipment under operating leases             26                                
Total rent expense under operating leases charged to operations             5,372 5,213 5,109                            
Number of landfill operation and management agreements                           1 3                
Depletion expense on operating lease contracts             9,372 8,482 7,878                            
Disputed invoice amount                     3,195                        
Aggregate finance asserting liability total                       3,852                      
Taxation payable                       2,220                      
Penalties and interest payable                       1,632                      
Payment of alleged actual or potential tax deficiencies             800                                
Payment allocated for tax             430                                
Payment allocated for interest             370                                
Scrap yard and solid waste transfer station                               25              
Undiscounted costs associated with implementing preferred remedies   10,219                                          
Present worth cost associated with implementing preferred remedies 12,130                                            
Enter into a Consent Order with DEC                   60                          
Environmental remediation charge     549 335 1,532 2,823     549                            
Percentage of environmental remediation cost         15.00%               30.00%                    
Environmental remediation liability             5,297 5,210                              
Accretion expense             138 138                              
Percentage of risk free interest rate             2.70% 2.70%                              
Future remediation costs                                     3,000 100      
Number of executive officers             4                                
Initial term of employment contract                                 3 years 1 year          
Covenant range                                 2 years 1 year          
Total annual commitments for salaries             $ 1,201                                
Salary and bonuses payment range                                 3 years 1 year