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Property, Plant and Equipment - Components of Property, Plant and Equipment (Detail) (USD $)
In Thousands, unless otherwise specified
Apr. 30, 2013
Apr. 30, 2012
Depreciation and Amortization Expenses For Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross $ 1,068,069 $ 1,005,899
Less: accumulated depreciation and amortization 645,567 591,233
Property, plant and equipment, net 422,502 414,666
Land [Member]
    
Depreciation and Amortization Expenses For Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 20,722 21,100
Landfills [Member]
    
Depreciation and Amortization Expenses For Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 475,855 445,812
Landfill operating lease contracts [Member]
    
Depreciation and Amortization Expenses For Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 109,363 103,103
Building and improvements [Member]
    
Depreciation and Amortization Expenses For Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 133,330 121,344
Machinery and equipment [Member]
    
Depreciation and Amortization Expenses For Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 120,314 116,091
Rolling stock [Member]
    
Depreciation and Amortization Expenses For Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross 128,038 125,033
Containers [Member]
    
Depreciation and Amortization Expenses For Property Plant And Equipment [Line Items]    
Property, plant and equipment, gross $ 80,447 $ 73,416