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Oil and Gas Properties (Changes in Asset Retirement Obligations) (Details) (USD $) In Thousands	6 Months Ended
	Jun. 30, 2011
Oil and Gas Properties
Asset retirement obligation at December 31, 2010	$ 234,936
Liability incurred during the period	672
Liability settled during the period	(25,273)
Revision in estimated cash flows	12,842
Accretion expense (included in depreciation and amortization)	7,630
Asset retirement obligations at June 30, 2011	$ 230,807

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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