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Asset Retirement Obligations (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Asset Retirement Obligation Disclosure [Abstract]
Beginning balance	$ 80,313	$ 65,155
Accretion	5,543	5,162
New obligations	15	2,257
Changes in estimates of existing obligations	(18,231)	8,116
Property Dispositions & Settlements	0	(377)
Ending balance	$ 67,640	$ 80,313

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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