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Summary of Significant Accounting Policies - Landfill Development Cost- Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Apr. 30, 2014 | Apr. 30, 2013 | Apr. 30, 2012
Summary Of Significant Accounting Policies [Line Items] |  |  |
Capitalized interest | $ 256 | $ 368 | $ 407
Estimate of inflation rate | 2.60% | 2.70% |
Accretion expense | 3,967 | 3,538 |
Landfills [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Accretion expense | 3,967 | 3,538 | 3,341
Surety bond | 133,847 | 128,551 |
Letters of credit | $ 1,104 | $ 1,752 |
Maximum [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Rate of asset retirement obligation | 9.50% |  |
Minimum [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Rate of asset retirement obligation | 9.00% |  |

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- Definition Amount of surety bonds that are available for the future payment of landfill capping, closure and post-closure obligations. + References No definition available. + Details
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- Definition Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Amount of interest capitalized during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Allowance for Funds Used during Construction -URI http://asc.fasb.org/extlink&oid=6504829 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391 + Details
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