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Changes in Asset Retirement Obligations (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Feb. 28, 2013	Feb. 29, 2012
Summary Of Significant Accounting Policies [Abstract]
Balance at beginning of period	$ 3,879	$ 4,455
Additions	219	287
Accretion expense	130	132
Settlements	(1,617)	(1,026)
Balance at end of period	$ 2,611	$ 3,848

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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