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Asset Retirement Obligations - Current and Non-current (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Asset Retirement Obligation Disclosure [Abstract]
Asset Retirement Obligation	$ 33,864	$ 34,113	$ 32,008	$ 33,880
Current portion	9,096	9,943	8,626
Long term portion	$ 24,768	$ 24,170	$ 23,382

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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