| **Variable Interest Entities (Details) (USD $)In Millions, unless otherwise specified**                                          | 3 Months Ended                                                                                                                                                                                                                                                | 0 Months Ended                                                                                                                                                                                                                                                                                                                   | 1 Months Ended                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                    |
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| 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 |   |

| X                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Variable Interest Entity, Financial or Other Support, Amount To Other Entity
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Variable Interest Entity, Financial or Other Support, Interest Rate Percentage
+ References
No definition available.
+ Details**Name:**gncma_VariableInterestEntityFinancialorOtherSupportInterestRatePercentage**Namespace Prefix:**gncma_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Variable Interest Entity, Percentage of Recapture
+ References
No definition available.
+ Details**Name:**gncma_VariableInterestEntityPercentageofRecapture**Namespace Prefix:**gncma_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Variable Interest Entity, Recapture Period
+ References
No definition available.
+ Details**Name:**gncma_VariableInterestEntityRecapturePeriod**Namespace Prefix:**gncma_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to investment tax credit.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 6.I) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319
+ Details**Name:**us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsInvestment**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.
+ References
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| - Definition
This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amounts due from parties associated with the reporting entity as evidenced by a written promise to pay, due within 1 year (or 1 business cycle).
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| - Definition
The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position.
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| - Definition
The carrying amount of the consolidated Variable Interest Entity's liabilities included in the reporting entity's statement of financial position.
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| - Definition
The amount of financial or other support the reporting entity has provided (explicitly or implicitly) to the Variable Interest Entity (VIE) that it was not previously contractually required to provide or that the reporting entity intends to provide, including, for example, situations in which the reporting entity assisted the VIE in obtaining another type of support.
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