Asset Retirement Obligations (Rollforward) (Details) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Asset Retirement Obligation Roll Forward Analysis [Roll Forward]
Beginning asset retirement obligation	$ 93,468,000	$ 85,744,000
Liabilities incurred and assumed during period	50,956,000	12,477,000
Revisions in estimated retirement obligations	5,334,000	12,217,000
Liabilities settled and sold during period	(50,556,000)	(23,257,000)
Accretion expense	7,228,000	6,287,000
Ending asset retirement obligation	106,430,000	93,468,000
Asset retirement obligations - current	(3,700,000)	(4,742,000)
Long-term asset retirement obligation	102,730,000	88,726,000
Asset Retirement Obligations (Textuals) [Abstract]
Balances in escrow accounts	$ 35,200,000	$ 34,100,000

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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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Carrying amount of an asset that is legally restricted for purposes of settling an asset retirement obligation.

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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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