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Asset Retirement Obligations - Schedule of Changes in Asset Retirement Obligations (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Change In Asset Retirement Obligation [Line Items]
Asset retirement obligation - beginning of period	$ 13,075	$ 7,640
Liabilities incurred during period	7,585	4,100
Liabilities settled during period	(71)	(369)
Accretion expense during period	872	590
Revisions to estimates	1,773	1,114
Asset retirement obligation - end of period	$ 23,234	$ 13,075

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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