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Asset Retirement Obligations (Schedule Of Changes In Asset Retirement Obligations) (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Change In Asset Retirement Obligation [Line Items]
Balance at December 31, 2011	$ 13,075
Liabilities incurred during period	5,388
Liabilities settled during period	(42)
Accretion expense during period (1)	612	[1]
Revisions to estimates	1,774
Balance at September 30, 2012	$ 20,807
[1] Included in depreciation, depletion and amortization on the Company's Condensed Consolidated Statement of Operations.

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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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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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