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Asset Retirement Obligation (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Change in the asset retirement obligation
Beginning balance	$ 64,019	$ 53,443
Liabilities incurred	2,993	917
Liabilities settled	(467)	(103)
Disposition of assets	(705)	0
Accretion expense	1,223	1,263
Revisions in estimated cash flows	2,001	0
Ending balance	$ 69,064	$ 55,520

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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