Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
ARO Operating Costs
Accretion	$ 8,195	$ 8,641	$ 8,802
Depreciation	7,242	11,516	13,732
Total	15,437	20,157	22,534
Asset Retirement Obligations
Balance at beginning of year	162,730	167,757
Liabilities incurred	1,738	2,501
Liabilities settled	(16,630)	(11,354)
Accretion expense	8,195	8,641	8,802
Revisions up (down), net	(2,054)	(4,815)
Balance at end of year	$ 153,979	$ 162,730	$ 167,757

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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