| **Variable Interest Entities (Details) - USD ($) $ in Millions**                                                                 |        |         |        | 1 Months Ended | 6 Months Ended |         |
|----------------------------------------------------------------------------------------------------------------------------------|--------|---------|--------|----------------|----------------|---------|
||
| Primary Beneficiary [Member]                                                                                                     |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Tax credit percentage                                                                                                            |   |    |   |           | 39.00%         |    |
| Percentage of recapture                                                                                                          |   |    |   |           | 100.00%        |    |
| Recapture period                                                                                                                 |   |    |   |           | 7 years        |    |
| Assets                                                                                                                           |   |    |   |           | $ 140.9        | $ 140.9 |
| Liabilities                                                                                                                      |   |    |   |           | $ 104.2        | $ 104.2 |
| NMTC 1 [Member] | Primary Beneficiary [Member]                                                                                   |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Variable Interest Entity, Financial or Other Support, Amount                                                                     |   |    | $ 58.3 |           |           |    |
| Interest rate percentage                                                                                                         |   |    | 1.00%  |           |           |    |
| NMTC 1 [Member] | Minimum [Member] | Primary Beneficiary [Member]                                                                |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Interest rate percentage                                                                                                         |   |    | 1.00%  |           |           |    |
| NMTC 1 [Member] | Maximum [Member] | Primary Beneficiary [Member]                                                                |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Interest rate percentage                                                                                                         |   |    | 3.96%  |           |           |    |
| NMTC 2 [Member] | Primary Beneficiary [Member]                                                                                   |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Variable Interest Entity, Financial or Other Support, Amount                                                                     |   | $ 37.7  |   |           |           |    |
| Interest rate percentage                                                                                                         |   | 1.00%   |   |           |           |    |
| NMTC 2 [Member] | Minimum [Member] | Primary Beneficiary [Member]                                                                |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Interest rate percentage                                                                                                         |   | 0.7099% |   |           |           |    |
| NMTC 2 [Member] | Maximum [Member] | Primary Beneficiary [Member]                                                                |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Interest rate percentage                                                                                                         |   | 0.7693% |   |           |           |    |
| NMTC 3 [Member] | Primary Beneficiary [Member]                                                                                   |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Variable Interest Entity, Financial or Other Support, Amount                                                                     | $ 8.2  |    |   |           |           |    |
| Interest rate percentage                                                                                                         | 1.00%  |    |   |           |           |    |
| NMTC 3 [Member] | Maximum [Member] | Primary Beneficiary [Member]                                                                |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Interest rate percentage                                                                                                         | 1.35%  |    |   |           |           |    |
| Next Generation Carrier-Class Communications Services Firm [Member]                                                              |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Ownership percentage                                                                                                             |   |    |   |           | 40.80%         |    |
| Next Generation Carrier-Class Communications Services Firm [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Variable Interest Entity, Financial or Other Support, Amount                                                                     |   |    |   | $ 3.0          |           |    |
| US Bancorp [Member] | NMTC 1 [Member] | Primary Beneficiary [Member]                                                             |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Amount to other entity                                                                                                           |   |    | $ 22.4 |           |           |    |
| US Bancorp [Member] | NMTC 2 [Member] | Primary Beneficiary [Member]                                                             |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Amount to other entity                                                                                                           |   | $ 17.5  |   |           |           |    |
| US Bancorp [Member] | NMTC 3 [Member] | Primary Beneficiary [Member]                                                             |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Amount to other entity                                                                                                           | $ 3.8  |    |   |           |           |    |
| Community Development Entities [Member] | NMTC 1 [Member] | Primary Beneficiary [Member]                                         |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Amount to other entity                                                                                                           |   |    | $ 76.8 |           |           |    |
| Community Development Entities [Member] | NMTC 2 [Member] | Primary Beneficiary [Member]                                         |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Amount to other entity                                                                                                           |   | $ 55.2  |   |           |           |    |
| Community Development Entities [Member] | NMTC 3 [Member] | Primary Beneficiary [Member]                                         |   |    |   |           |           |    |
| **Variable Interest Entity [Line Items]**                                                                                        |   |    |   |           |           |    |
| Amount to other entity                                                                                                           | $ 12.0 |    |   |           |           |    |

| X                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Investment Tax Credit Percent
+ References
No definition available.
+ Details**Name:**gncma_InvestmentTaxCreditPercent**Namespace Prefix:**gncma_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Variable Interest Entity, Financial or Other Support, Amount To Other Entity
+ References
No definition available.
+ Details**Name:**gncma_VariableInterestEntityFinancialorOtherSupportAmountToOtherEntity**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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
The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571
+ Details**Name:**us-gaap_EquityMethodInvestmentOwnershipPercentage**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=64801991&loc=d3e5710-111685
+ Details**Name:**us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The carrying amount of the consolidated Variable Interest Entity's liabilities included in the reporting entity's statement of financial position.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=64801991&loc=d3e5710-111685
+ Details**Name:**us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 5A -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=64801991&loc=SL6759159-111685
+ Details**Name:**us-gaap_VariableInterestEntityFinancialOrOtherSupportAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_VariableInterestEntityLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_VariableInterestEntityPrimaryBeneficiaryMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_TypeOfArrangementAxis=gncma_NMTCOneMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MinimumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MaximumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_TypeOfArrangementAxis=gncma_NMTCTwoMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_TypeOfArrangementAxis=gncma_NMTCThreeMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=gncma_NextGenerationCarrierClassCommunicationsServicesFirmMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis=gncma_USBancorpMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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