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Asset Retirement Obligations (Details) (USD $)	3 Months Ended		6 Months Ended
Asset Retirement Obligations (Details) (USD $)	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Changes in Asset Retirement Obligations
Asset retirement obligations as of the beginning of the period, including current portion			$ 425,800,000
Liabilities settled			(21,900,000)
Liabilities assumed			14,500,000
Divestment of Properties			(7,600,000)
Accretion expense	8,255,000	7,717,000	16,521,000	15,434,000
Asset retirement obligations as of the end of the year, including current portion	$ 427,300,000		$ 427,300,000

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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