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Asset Retirement Obligation (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Change in the asset retirement obligation
Beginning balance	$ 64,019	$ 53,443
Accretion expense	4,201	3,593
Revisions in estimated cash flows	10,992	4,308
Ending balance	84,257	61,711
Developmental Activities
Change in the asset retirement obligation
Liabilities incurred/assumed	5,862	2,169
Liabilities Settled
Change in the asset retirement obligation
Liabilities settled	(2,763)	(1,921)
Acquisition Activities
Change in the asset retirement obligation
Liabilities incurred/assumed	2,651	119
Divestitures Activities
Change in the asset retirement obligation
Liabilities settled	$ (705)	$ 0

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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