Asset Retirement Obligations (Details) (USD $) In Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
ARO Operating Costs
Accretion	$ 2,124	$ 2,255	$ 4,296	$ 4,444
Depreciation	1,853	3,157	3,395	6,340
Total	3,977	5,412	7,691	10,784
Asset Retirement Obligations
Balance at beginning of period	162,591	163,931	162,730	167,757
Liabilities incurred	278	1,441	278	1,441
Liabilities settled	(3,632)	1,740	(5,964)	4,117
Accretion expense	2,124	2,255	4,296	4,444
Revisions up (down)	(628)	(3,719)	(607)	(7,357)
Balance at end of period	$ 160,733	$ 162,168	$ 160,733	$ 162,168

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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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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- Definition

Amount of asset retirement obligations (ARO) costs incurred (whether capitalized or charged to expense) during the period.

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