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Asset Retirement Obligations (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Changes in the Company's asset retirement obligations      
Beginning asset retirement obligations $ 11,951us-gaap_AssetRetirementObligation    
Liabilities incurred during period 2,404us-gaap_AssetRetirementObligationLiabilitiesIncurred    
Liabilities settled during period (221)us-gaap_AssetRetirementObligationLiabilitiesSettled    
Revisions in estimated cash flows (703)us-gaap_AssetRetirementObligationRevisionOfEstimate    
Accretion expense 112us-gaap_AssetRetirementObligationAccretionExpense 117us-gaap_AssetRetirementObligationAccretionExpense  
Ending asset retirement obligations 13,543us-gaap_AssetRetirementObligation    
Less: current asset retirement obligations (268)us-gaap_AssetRetirementObligationCurrent [1]    
Long-term asset retirement obligations $ 13,275us-gaap_AssetRetirementObligationsNoncurrent   $ 11,640us-gaap_AssetRetirementObligationsNoncurrent
[1] Included in accrued liabilities in the Company’s unaudited condensed consolidated balance sheet at March 31, 2015.