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Information Related to Asset Retirement Obligation (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Dec. 31, 2010
Information related to asset retirement obligation
Beginning balance	$ 180,662	$ 173,673	$ 160,260
Revisions in cash flow estimates			7,809
Additions to asset retirement obligations	649	708	261
Accretion expense	8,061	10,573	10,751
Liabilities settled	(3,531)	(4,292)	(5,408)
Ending balance	$ 185,841	$ 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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