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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	9 Months Ended
	Jan. 31, 2013	Jan. 31, 2012
Asset Retirement Obligations [Line Items]
Beginning balance	$ 39,629	$ 36,407
Obligations incurred	2,606	2,350
Accretion expense	2,653	2,510
Payments	(2,304)	(1,298)
Ending balance	$ 42,584	$ 39,969

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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- 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.

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