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Asset Retirement Obligation - Change in Asset Retirement Obligation (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Asset Retirement Obligation Disclosure [Abstract]    
Asset retirement obligation at beginning of period $ 2us-gaap_AssetRetirementObligation $ 4us-gaap_AssetRetirementObligation
Additions during the period    1us-gaap_AssetRetirementObligationLiabilitiesIncurred
Revisions of estimates    (3)us-gaap_AssetRetirementObligationRevisionOfEstimate
Asset retirement obligation at end of period $ 2us-gaap_AssetRetirementObligation [1] $ 2us-gaap_AssetRetirementObligation
[1] This is a recurring Level 3 fair value measurement.