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Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	$ 46,492	$ 41,695
Liabilities incurred in current period	551	396
Liabilities settled in current period	(1,484)	(756)
Accretion expense	2,962	2,683
Ending balance	48,521	44,018
Asset Retirement Obligation, Current	2,900	1,350
Asset Retirement Obligations, Noncurrent	$ 45,621	$ 42,668	$ 43,593

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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