Quarterly report pursuant to Section 13 or 15(d)

Variable Interest Entities (Details)

v2.4.1.9
Variable Interest Entities (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 0 Months Ended 1 Months Ended
Mar. 31, 2015
Aug. 30, 2011
Oct. 03, 2012
Dec. 11, 2012
Mar. 31, 2015
Dec. 31, 2014
Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Project fund $ 59.3us-gaap_VariableInterestEntityFinancialOrOtherSupportAmount
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
         
Tax credit percentage 39.00%us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsInvestment
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
         
Percentage of recapture 100.00%gncma_VariableInterestEntityPercentageofRecapture
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
         
Recapture period 7 years          
Assets 140.9us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
      140.9us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
140.9us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Liabilities 104.2us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
      104.2us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
104.2us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
NMTC 1 [Member] | Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Project fund   58.3us-gaap_VariableInterestEntityFinancialOrOtherSupportAmount
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCOneMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
       
Interest rate percentage   1.00%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCOneMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
       
NMTC 1 [Member] | Minimum [Member] | Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Interest rate percentage   1.00%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCOneMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
       
NMTC 1 [Member] | Maximum [Member] | Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Interest rate percentage   3.96%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCOneMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
       
NMTC 2 [Member] | Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Project fund     37.7us-gaap_VariableInterestEntityFinancialOrOtherSupportAmount
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCTwoMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
     
Interest rate percentage     1.00%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCTwoMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
     
NMTC 2 [Member] | Minimum [Member] | Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Interest rate percentage     0.7099%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCTwoMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
     
NMTC 2 [Member] | Maximum [Member] | Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Interest rate percentage     0.7693%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCTwoMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
     
NMTC 3 [Member] | Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Project fund       8.2us-gaap_VariableInterestEntityFinancialOrOtherSupportAmount
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCThreeMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
   
Interest rate percentage       1.00%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCThreeMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
   
NMTC 3 [Member] | Maximum [Member] | Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Interest rate percentage       1.35%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCThreeMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
   
US Bancorp [Member] | NMTC 1 [Member] | Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Amount to other entity   22.4gncma_VariableInterestEntityFinancialorOtherSupportAmountToOtherEntity
/ us-gaap_LeaseArrangementTypeAxis
= gncma_USBancorpMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCOneMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
       
US Bancorp [Member] | NMTC 2 [Member] | Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Amount to other entity     17.5gncma_VariableInterestEntityFinancialorOtherSupportAmountToOtherEntity
/ us-gaap_LeaseArrangementTypeAxis
= gncma_USBancorpMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCTwoMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
     
US Bancorp [Member] | NMTC 3 [Member] | Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Amount to other entity       3.8gncma_VariableInterestEntityFinancialorOtherSupportAmountToOtherEntity
/ us-gaap_LeaseArrangementTypeAxis
= gncma_USBancorpMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCThreeMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
   
Community Development Entities [Member] | NMTC 1 [Member] | Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Amount to other entity   76.8gncma_VariableInterestEntityFinancialorOtherSupportAmountToOtherEntity
/ us-gaap_LeaseArrangementTypeAxis
= gncma_CommunityDevelopmentEntitiesMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCOneMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
       
Community Development Entities [Member] | NMTC 2 [Member] | Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Amount to other entity     55.2gncma_VariableInterestEntityFinancialorOtherSupportAmountToOtherEntity
/ us-gaap_LeaseArrangementTypeAxis
= gncma_CommunityDevelopmentEntitiesMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCTwoMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
     
Community Development Entities [Member] | NMTC 3 [Member] | Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Amount to other entity       12.0gncma_VariableInterestEntityFinancialorOtherSupportAmountToOtherEntity
/ us-gaap_LeaseArrangementTypeAxis
= gncma_CommunityDevelopmentEntitiesMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCThreeMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
   
Next Generation Carrier-Class Communications Services Firm [Member]            
Variable Interest Entity [Line Items]            
Ownership percentage 40.80%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gncma_NextGenerationCarrierClassCommunicationsServicesFirmMember
      40.80%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gncma_NextGenerationCarrierClassCommunicationsServicesFirmMember
 
Next Generation Carrier-Class Communications Services Firm [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]            
Variable Interest Entity [Line Items]            
Project fund         3.0us-gaap_VariableInterestEntityFinancialOrOtherSupportAmount
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gncma_NextGenerationCarrierClassCommunicationsServicesFirmMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember
 
Other Current Assets [Member] | Next Generation Carrier-Class Communications Services Firm [Member]            
Variable Interest Entity [Line Items]            
Cost method Investment 13.1us-gaap_EquityMethodInvestments
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentAssetsMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gncma_NextGenerationCarrierClassCommunicationsServicesFirmMember
      13.1us-gaap_EquityMethodInvestments
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentAssetsMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gncma_NextGenerationCarrierClassCommunicationsServicesFirmMember
 
Notes receivable $ 3.0us-gaap_NotesReceivableRelatedPartiesCurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentAssetsMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gncma_NextGenerationCarrierClassCommunicationsServicesFirmMember
      $ 3.0us-gaap_NotesReceivableRelatedPartiesCurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentAssetsMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gncma_NextGenerationCarrierClassCommunicationsServicesFirmMember