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Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	$ 49,214	$ 46,493
Liabilities incurred in current period	262	416
Liabilities settled in current period	(2,867)	(1,211)
Accretion expense	2,008	1,969
Ending balance	48,617	47,667
Less current portion	2,900	2,900
Asset retirement obligations, net of current portion	$ 45,717	$ 44,767	$ 46,314

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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