Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2012	Mar. 31, 2011
ARO Operating Costs
Accretion	$ 2,019	$ 2,172
Depreciation	1,864	1,541
Total	3,883	3,713
Asset Retirement Obligations
Balance at beginning of period	153,979	162,730
Liabilities incurred	0	0
Liabilities settled	(621)	(2,332)
Accretion expense	2,019	2,172
Revisions up (down), net	25	21
Balance at end of period	$ 155,402	$ 162,591

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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