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Asset Retirement Obligations (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
AssetRetirementObligationDisclosureAbstract
Beginning balance	$ 302,768	$ 251,644
Accretion	23,052	19,866
New obligations	2,556	530
Changes in estimates of existing obligations	10,895	74,039
Property dispositions/obligations settled	(42,401)	(43,311)
Ending balance	$ 296,870	$ 302,768

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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