Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements and Derivative Instruments (Carrying amounts and fair value of the financial instruments) (Details)

v2.4.1.9
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