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Asset Retirement Obligations Operating Costs (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
ARO Operating Costs
Accretion	$ 7,956	$ 8,195	$ 8,641
Depreciation	5,599	7,242	11,516
Total	$ 13,555	$ 15,437	$ 20,157

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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- Definition

Amount of asset retirement obligations (ARO) costs incurred (whether capitalized or charged to expense) during the period.

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