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Commitments and Contingencies - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended 3 Months Ended 12 Months Ended 12 Months Ended
Apr. 30, 2014
Executives
Apr. 30, 2013
Apr. 30, 2012
Apr. 30, 2011
Potsdam Environmental Liability [Member]
Apr. 30, 2010
Potsdam Environmental Liability [Member]
Apr. 30, 2009
Potsdam Environmental Liability [Member]
Jan. 31, 2009
Potsdam Environmental Liability [Member]
Apr. 30, 2014
Potsdam Environmental Liability [Member]
acre
Apr. 30, 2013
Potsdam Environmental Liability [Member]
May 31, 2009
Potsdam Environmental Liability [Member]
Sep. 30, 2011
Potsdam Environmental Liability [Member]
Waste-Stream Inc [Member]
Nov. 30, 2011
Potsdam Environmental Liability [Member]
Motors Liquidation Trust [Member]
Apr. 30, 2014
Southbridge Landfill [Member]
Condition One [Member]
Apr. 30, 2014
Maximum [Member]
Apr. 30, 2014
Maximum [Member]
Employment Contracts [Member]
Apr. 30, 2014
Minimum [Member]
Apr. 30, 2014
Minimum [Member]
Employment Contracts [Member]
Contingencies And Commitments [Line Items]                                  
Operating facilities and equipment under operating leases                           $ 26      
Total rent expense under operating leases charged to operations 5,651 5,372 5,213                            
Scrap yard and solid waste transfer station               25                  
Estimate of total undiscounted costs associated with implementing the preferred remedies       12,130           10,219              
Environmental remediation charge 400     549 335 1,532 2,823           400        
Percentage of environmental remediation cost           15.00% 30.00%                    
Environmental remediation liability 5,320             5,320 5,297                
Accretion expense               19 138                
Estimate of inflation rate 2.60% 2.70%           2.00%                  
Future remediation costs                     3,000 100          
Anticipated remediation costs                         2,100        
Loss contingency range of possible loss minimum                         400        
Loss contingency range of possible loss maximum                         2,100        
Number of executive officers 4                                
Initial term of employment contract                           3 years   1 year  
Intangible assets, useful life                           10 years 2 years 5 years 1 year
Contract extension period 1 year                                
Total annual commitments for salaries $ 1,397                                
Salary and bonuses payment range                           3 years   1 year