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Changes in Asset Retirement Obligations (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	May 31, 2012	May 31, 2011
Summary Of Significant Accounting Policies [Abstract]
Balance at beginning of period	$ 4,455	$ 4,474
Additions	287	441
Accretion expense	175	194
Settlements	(1,038)	(654)
Balance at end of period	$ 3,879	$ 4,455

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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