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Equity Method Investments (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Constitution Pipeline Company, LLC    
Equity Method Investment    
Contributions made to equity method investments $ 3.0cog_JointlyOwnedPipelineNetOwnershipContributions
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cog_ConstitutionPipelineCompanyLLCMember		 $ 5.8cog_JointlyOwnedPipelineNetOwnershipContributions
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cog_ConstitutionPipelineCompanyLLCMember
Meade Pipeline Co LLC    
Equity Method Investment    
Contributions made to equity method investments $ 2.1cog_JointlyOwnedPipelineNetOwnershipContributions
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cog_MeadePipelineCoLLCMember		 $ 0.2cog_JointlyOwnedPipelineNetOwnershipContributions
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cog_MeadePipelineCoLLCMember