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Property, Plant and Equipment - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Apr. 30, 2013 | Apr. 30, 2012 | Apr. 30, 2011
Depreciation and Amortization Expenses For Property Plant And Equipment [Line Items] |  |  |
Depreciation expense | $ 34,065 | $ 37,829 | $ 35,939
Depletion expense on operating lease contracts | 9,372 | 8,482 | 7,878
Landfills [Member] |  |  |
Depreciation and Amortization Expenses For Property Plant And Equipment [Line Items] |  |  |
Amortization expense | 21,206 | 19,957 | 21,342
Depletion expense on operating lease contracts | $ 9,372 | $ 8,482 | $ 7,878

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- Definition The aggregate amount of recurring noncash expense charged against earnings in the period to allocate the cost of assets over their estimated remaining economic lives. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 + Details
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- Definition The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 5 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 + Details
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