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Asset Retirement Obligations - Current and Non-current (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010
Asset Retirement Obligation Disclosure [Abstract]
Asset Retirement Obligation	$ 30,866	$ 33,880	$ 39,988	$ 44,952
Current portion	8,256	8,996	15,459
Long term portion	$ 22,610	$ 24,884	$ 24,529

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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