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Summary of Significant Accounting Policies - Changes in Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	May 31, 2013	May 31, 2012
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of period	$ 3,879	$ 4,455
Additions	80	287
Accretion expense	175	175
Settlements	(1,481)	(1,038)
Balance at end of period	$ 2,653	$ 3,879

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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