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Reconciliations of Asset Retirement Obligations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Asset Retirement Obligations
Balance at beginning of period	$ 150,072	$ 153,979
Liabilities incurred	0	0
Liabilities settled	(1,292)	(621)
Accretion expense	2,006	2,019
Revisions up (down), net	5,672	25
Balance at end of period	$ 156,458	$ 155,402

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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