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Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Changes in asset retirement obligations
Beginning of period	$ 40,794	$ 40,444
Additional ARO from new properties	7,868	1,526
Sales or abandonments of properties	(2,184)	(4,425)
Accretion expense	3,696	2,757	2,623
Revisions of previous estimates	1,303	492
End of period	$ 51,477	$ 40,794	$ 40,444

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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