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Changes in Aggregate ARO (Detail) (USD $) In Thousands	3 Months Ended		6 Months Ended
Changes in Aggregate ARO (Detail) (USD $) In Thousands	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Reconciliation of Changes in Asset Retirement Obligations [Line Items]
Balance at beginning of period			$ 68,430
Liabilities incurred			311
Accretion expense	970	764	1,936	1,274
Revisions in estimated cash flows			3,118
Settlements and divestitures			(4,275)
Balance at end of period	$ 69,520		$ 69,520

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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