Annual report pursuant to Section 13 and 15(d)

Variable Interest Entities (Details)

v2.4.1.9
Variable Interest Entities (Details) (Primary Beneficiary, USD $)
In Millions, unless otherwise specified
12 Months Ended 0 Months Ended
Dec. 31, 2014
Aug. 30, 2011
Oct. 03, 2012
Dec. 11, 2012
Dec. 31, 2013
Variable Interest Entity [Line Items]          
Financial support $ 59.3us-gaap_VariableInterestEntityFinancialOrOtherSupportAmount        
Tax credit percentage 39.00%us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsInvestment        
Restricted cash 1.1us-gaap_RestrictedCashAndCashEquivalents       6.9us-gaap_RestrictedCashAndCashEquivalents
Percentage of recapture 100.00%gncma_VariableInterestEntityPercentageofRecapture        
Recapture period 7 years        
Assets 140.9us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets       140.9us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets
Liabilities 104.2us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities       104.2us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities
NMTC 1          
Variable Interest Entity [Line Items]          
Financial support   58.3us-gaap_VariableInterestEntityFinancialOrOtherSupportAmount
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCOneMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
     
Interest rate percent on financial support   1.00%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCOneMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
     
NMTC 1 | Minimum          
Variable Interest Entity [Line Items]          
Interest rate percent on financial support   1.00%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCOneMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
     
NMTC 1 | Maximum          
Variable Interest Entity [Line Items]          
Interest rate percent on financial support   3.96%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCOneMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
     
NMTC 1 | US Bancorp          
Variable Interest Entity [Line Items]          
Financial support to other entity   22.4gncma_VariableInterestEntityFinancialorOtherSupportAmountToOtherEntity
/ us-gaap_LeaseArrangementTypeAxis
= gncma_USBancorpMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCOneMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
     
NMTC 1 | CDE's          
Variable Interest Entity [Line Items]          
Financial support to other entity   76.8gncma_VariableInterestEntityFinancialorOtherSupportAmountToOtherEntity
/ us-gaap_LeaseArrangementTypeAxis
= gncma_CommunityDevelopmentEntitiesMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCOneMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
     
NMTC 2          
Variable Interest Entity [Line Items]          
Financial support     37.7us-gaap_VariableInterestEntityFinancialOrOtherSupportAmount
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCTwoMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
   
Interest rate percent on financial support     1.00%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCTwoMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
   
NMTC 2 | Minimum          
Variable Interest Entity [Line Items]          
Interest rate percent on financial support     0.7099%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCTwoMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
   
NMTC 2 | Maximum          
Variable Interest Entity [Line Items]          
Interest rate percent on financial support     0.7693%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCTwoMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
   
NMTC 2 | US Bancorp          
Variable Interest Entity [Line Items]          
Financial support to other entity     17.5gncma_VariableInterestEntityFinancialorOtherSupportAmountToOtherEntity
/ us-gaap_LeaseArrangementTypeAxis
= gncma_USBancorpMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCTwoMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
   
NMTC 2 | CDE's          
Variable Interest Entity [Line Items]          
Financial support to other entity     55.2gncma_VariableInterestEntityFinancialorOtherSupportAmountToOtherEntity
/ us-gaap_LeaseArrangementTypeAxis
= gncma_CommunityDevelopmentEntitiesMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCTwoMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
   
NMTC 3          
Variable Interest Entity [Line Items]          
Financial support       8.2us-gaap_VariableInterestEntityFinancialOrOtherSupportAmount
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCThreeMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
 
Interest rate percent on financial support       1.00%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCThreeMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
 
NMTC 3 | Maximum          
Variable Interest Entity [Line Items]          
Interest rate percent on financial support       1.35%gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCThreeMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
 
NMTC 3 | US Bancorp          
Variable Interest Entity [Line Items]          
Financial support to other entity       3.8gncma_VariableInterestEntityFinancialorOtherSupportAmountToOtherEntity
/ us-gaap_LeaseArrangementTypeAxis
= gncma_USBancorpMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCThreeMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
 
NMTC 3 | CDE's          
Variable Interest Entity [Line Items]          
Financial support to other entity       $ 12.0gncma_VariableInterestEntityFinancialorOtherSupportAmountToOtherEntity
/ us-gaap_LeaseArrangementTypeAxis
= gncma_CommunityDevelopmentEntitiesMember
/ us-gaap_TypeOfArrangementAxis
= gncma_NMTCThreeMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember