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Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Changes in the Company's asset retirement obligations
Beginning asset retirement obligations	$ 4,270	$ 3,695
Liabilities incurred during period	1,243	187
Revisions in estimated cash flows		312
Liabilities settled during period		(133)
Accretion expense	256	209	155
Ending asset retirement obligations	$ 5,769	$ 4,270	$ 3,695

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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