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Asset Retirement Obligations Operating Costs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
ARO Operating Costs
Accretion	$ 2,006	$ 2,019
Depreciation	779	1,864
Total	$ 2,785	$ 3,883

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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