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Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Changes in carrying amount of asset retirement obligations
Balance at the beginning of the period	$ 240,634
Accretion	4,127	2,649
Revisions to estimated cash flows	(2,057)
Payments	(327)	(1,466)
Balance at the end of the period	242,377
Less current portion	(3,693)
Asset retirement obligation, net of current portion	$ 238,684		$ 238,991

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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