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Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Changes in Company's asset retirement obligations
Asset retirement obligations at the beginning of the period	$ 15,245
Liabilities incurred	207
Current period accretion expense	254	134
Asset retirement obligations at the end of the period	$ 15,706

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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