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Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Asset retirement obligations:
Beginning asset retirement obligations	$ 27,484
Liabilities incurred during period	1,676
Accretion expense	797
Ending asset retirement obligations	$ 29,957

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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