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Asset Retirement Obligation (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Information related to asset retirement obligation
Balance at December 31, 2011	$ 180,662	$ 173,673	$ 160,260
Revisions in cash flow estimates			7,809
Additions to asset retirement obligations	155	708	261
Accretion expense	1,976	10,573	10,751
Liabilities settled	(1,338)	(4,292)	(5,408)
Balance at March 31, 2012	$ 181,455	$ 180,662	$ 173,673

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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