Annual report pursuant to Section 13 and 15(d)

Long Term Debt (Schedule of Redemption Prices) (Details)

v2.4.1.9
Long Term Debt (Schedule of Redemption Prices) (Details) (Line of Credit)
 12 Months Ended
Dec. 31, 2014
2021 Notes | Redemption period one  
Debt Instrument [Line Items]  
Redemption price, percentage 103.375%us-gaap_DebtInstrumentRedemptionPricePercentage
/ us-gaap_DebtInstrumentAxis
= gncma_SeniorNotesDueTwentyTwentyOneMember
/ us-gaap_DebtInstrumentRedemptionPeriodAxis
= us-gaap_DebtInstrumentRedemptionPeriodOneMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember
2021 Notes | Redemption period two  
Debt Instrument [Line Items]  
Redemption price, percentage 102.25%us-gaap_DebtInstrumentRedemptionPricePercentage
/ us-gaap_DebtInstrumentAxis
= gncma_SeniorNotesDueTwentyTwentyOneMember
/ us-gaap_DebtInstrumentRedemptionPeriodAxis
= us-gaap_DebtInstrumentRedemptionPeriodTwoMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember
2021 Notes | Redemption period three  
Debt Instrument [Line Items]  
Redemption price, percentage 101.125%us-gaap_DebtInstrumentRedemptionPricePercentage
/ us-gaap_DebtInstrumentAxis
= gncma_SeniorNotesDueTwentyTwentyOneMember
/ us-gaap_DebtInstrumentRedemptionPeriodAxis
= us-gaap_DebtInstrumentRedemptionPeriodThreeMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember
2021 Notes | Redemption period four  
Debt Instrument [Line Items]  
Redemption price, percentage 100.00%us-gaap_DebtInstrumentRedemptionPricePercentage
/ us-gaap_DebtInstrumentAxis
= gncma_SeniorNotesDueTwentyTwentyOneMember
/ us-gaap_DebtInstrumentRedemptionPeriodAxis
= us-gaap_DebtInstrumentRedemptionPeriodFourMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember
2019 Notes | Redemption period one  
Debt Instrument [Line Items]  
Redemption price, percentage 104.313%us-gaap_DebtInstrumentRedemptionPricePercentage
/ us-gaap_DebtInstrumentAxis
= gncma_SeniorNotesDueTwentyNineteenMember
/ us-gaap_DebtInstrumentRedemptionPeriodAxis
= us-gaap_DebtInstrumentRedemptionPeriodOneMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember
2019 Notes | Redemption period two  
Debt Instrument [Line Items]  
Redemption price, percentage 103.00%us-gaap_DebtInstrumentRedemptionPricePercentage
/ us-gaap_DebtInstrumentAxis
= gncma_SeniorNotesDueTwentyNineteenMember
/ us-gaap_DebtInstrumentRedemptionPeriodAxis
= us-gaap_DebtInstrumentRedemptionPeriodTwoMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember
2019 Notes | Redemption period three  
Debt Instrument [Line Items]  
Redemption price, percentage 101.438%us-gaap_DebtInstrumentRedemptionPricePercentage
/ us-gaap_DebtInstrumentAxis
= gncma_SeniorNotesDueTwentyNineteenMember
/ us-gaap_DebtInstrumentRedemptionPeriodAxis
= us-gaap_DebtInstrumentRedemptionPeriodThreeMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember
2019 Notes | Redemption period four  
Debt Instrument [Line Items]  
Redemption price, percentage 100.00%us-gaap_DebtInstrumentRedemptionPricePercentage
/ us-gaap_DebtInstrumentAxis
= gncma_SeniorNotesDueTwentyNineteenMember
/ us-gaap_DebtInstrumentRedemptionPeriodAxis
= us-gaap_DebtInstrumentRedemptionPeriodFourMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember