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Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Changes in Asset Retirement Obligations
Asset retirement obligations as of the beginning of the year, including current portion	$ 373,920	$ 320,599	$ 275,628
Liabilities incurred	7,993	1,528	3,035
Liabilities settled	(63,226)	(36,392)	(67,858)
Liabilities assumed	59,441
Divestment of properties	(10,900)	(692)	(5,941)
Accretion expense	30,764	34,469	39,306
Revision estimates	27,787	54,408	76,429
Asset retirement obligations as of the end of the year, including current portion	$ 425,779	$ 373,920	$ 320,599

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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