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Asset Retirement Obligation (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Asset Retirement Obligation    
Balance at beginning of period $ 75,853,000us-gaap_AssetRetirementObligation  
Liabilities incurred 7,220,000us-gaap_AssetRetirementObligationLiabilitiesIncurred  
Liabilities settled and divested (1,474,000)us-gaap_AssetRetirementObligationLiabilitiesSettled  
Liabilities acquired 1,206,000cog_AssetRetirementObligationLiabilitiesAcquired  
Accretion expense 5,110,000us-gaap_AssetRetirementObligationAccretionExpense  
Change in estimate 38,740,000us-gaap_AssetRetirementObligationRevisionOfEstimate  
Balance at end of period 126,655,000us-gaap_AssetRetirementObligation  
Additional disclosures    
Asset retirement obligation, current $ 2,000,000us-gaap_AssetRetirementObligationCurrent $ 2,000,000us-gaap_AssetRetirementObligationCurrent