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Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Change in the asset retirement obligations
Beginning balance	$ 86,746	$ 64,019
Accretion expense	1,704	1,223
Revisions in estimated cash flows	4,311	2,001
Ending balance	94,667	69,064
Developmental Activities
Change in the asset retirement obligations
Liabilities incurred/assumed	2,711	2,993
Liabilities Settled
Change in the asset retirement obligations
Liabilities settled	(805)	(467)
Divestitures Activities
Change in the asset retirement obligations
Liabilities settled	$ 0	$ (705)

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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