Summary of Significant Accounting Policies - Landfill Development Cost- Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
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%

Estimated inflation rate.

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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

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Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bbb)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

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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

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

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