Asset Retirement Obligations (Schedule of Changes in Asset Retirement Obligations) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Beginning of period total	$ 99,347	$ 71,583
Accretion expense	15,565	15,942	12,845
Liabilities assumed in acquisition	132,109	26,435
Liabilities incurred	1,210	157
Revisions	22,308	17,595
Liabilities settled	(35,429)	(32,365)
End of period total	235,110	99,347	71,583
Less: End of period, current portion	(30,179)	(25,578)
End of period, noncurrent portion	204,931	73,769
Parent Company [Member]
Accretion expense	10,551	9,013	6,018
Less: End of period, current portion	(23,982)	(16,178)
End of period, noncurrent portion	$ 187,790	$ 50,805

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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Current portion of 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 asset retirement obligations incurred during the period.

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Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

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Amount of the Increase or Decrease in the amount of the asset retirement obligation during the current period for changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

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Noncurrent portion of 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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The amount of acquisition cost of a business combination allocated to liabilities assumed.

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