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Accrued Capping, Closure and Post Closure - Changes to Accrued Capping, Closure and Post-Closure Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Apr. 30, 2013	Apr. 30, 2012
Asset Retirement Obligations [Line Items]
Beginning balance	$ 39,629	$ 36,407
Obligations incurred	3,188	3,123
Revisions in estimates	(694)	(1,682)
Accretion expense	3,538	3,341
Payments	(2,491)	(1,560)
Ending balance	$ 43,170	$ 39,629

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The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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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.

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Amount of cash paid during the period to settle an asset retirement obligation. Amounts paid to settle an asset retirement obligation are generally included in the operating section of the Statement of Cash Flows.

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Amount of asset retirement obligations incurred during the period.

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Amount of the Increase or Decrease in the amount of the asset retirement obligation during the current period for changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

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