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Fair Value Measurements and Derivative Instruments (Carrying amounts and fair value of the financial instruments) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Senior Notes Due 2021 | Line of Credit    
Debt Instrument [Line Items]    
Debt stated percentage 6.75%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= gncma_SeniorNotesDueTwentyTwentyOneMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember		  
Senior Notes Due 2019 | Line of Credit    
Debt Instrument [Line Items]    
Debt stated percentage 8.63%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= gncma_SeniorNotesDueTwentyNineteenMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember		  
Carrying Amount    
Debt Instrument [Line Items]    
Current and long-term debt $ 1,326,172us-gaap_DebtInstrumentFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		 $ 1,036,678us-gaap_DebtInstrumentFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Fair Value    
Debt Instrument [Line Items]    
Current and long-term debt $ 1,376,834us-gaap_DebtInstrumentFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember		 $ 1,055,952us-gaap_DebtInstrumentFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember