| **Net Property and Equipment in Service (Details) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                                                                                  | Dec. 31, 2013                                                                                                                                                                                  |
|-----------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Property, Plant and Equipment [Line Items]**                                                      |                                                                                                                                                                                           |                                                                                                                                                                                           |
| Property, plant and equipment, gross                                                                | $ 2,242,271gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress                                                                                                                 | $ 2,053,854gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress                                                                                                                 |
| Less accumulated depreciation                                                                       | 1,178,982gncma_AccumulatedDepreciationPropertyPlantandEquipment                                                                                                                                | 1,042,724gncma_AccumulatedDepreciationPropertyPlantandEquipment                                                                                                                                |
| Less accumulated amortization                                                                       | 50,047gncma_AccumulatedAmortizationPropertyPlantandEquipment                                                                                                                                   | 41,552gncma_AccumulatedAmortizationPropertyPlantandEquipment                                                                                                                                   |
| Net property and equipment in service                                                               | 1,013,242gncma_PropertyandEquipmentinServiceNet                                                                                                                                                | 969,578gncma_PropertyandEquipmentinServiceNet                                                                                                                                                  |
| Property and equipment under capital leases                                                         | 112,495us-gaap_CapitalLeasedAssetsGross                                                                                                                                                        | 104,251us-gaap_CapitalLeasedAssetsGross                                                                                                                                                        |
| Land and buildings                                                                                  |                                                                                                                                                                                           |                                                                                                                                                                                           |
| **Property, Plant and Equipment [Line Items]**                                                      |                                                                                                                                                                                           |                                                                                                                                                                                           |
| Property, plant and equipment, gross                                                                | 100,038gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= us-gaap_LandAndBuildingMember                                         | 69,984gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= us-gaap_LandAndBuildingMember                                          |
| Telephony transmission equipment and distribution facilities                                        |                                                                                                                                                                                           |                                                                                                                                                                                           |
| **Property, Plant and Equipment [Line Items]**                                                      |                                                                                                                                                                                           |                                                                                                                                                                                           |
| Property, plant and equipment, gross                                                                | 1,189,470gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= gncma_TelephonyTransmissionEquipmentAndDistributionFacilitiesMember | 1,085,963gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= gncma_TelephonyTransmissionEquipmentAndDistributionFacilitiesMember |
| Cable transmission equipment and distribution facilities                                            |                                                                                                                                                                                           |                                                                                                                                                                                           |
| **Property, Plant and Equipment [Line Items]**                                                      |                                                                                                                                                                                           |                                                                                                                                                                                           |
| Property, plant and equipment, gross                                                                | 193,832gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= gncma_CableTransmissionEquipmentandDistributionFacilitiesMember       | 177,410gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= gncma_CableTransmissionEquipmentandDistributionFacilitiesMember       |
| Studio equipment                                                                                    |                                                                                                                                                                                           |                                                                                                                                                                                           |
| **Property, Plant and Equipment [Line Items]**                                                      |                                                                                                                                                                                           |                                                                                                                                                                                           |
| Property, plant and equipment, gross                                                                | 14,396gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= gncma_StudioEquipmentMember                                            | 12,680gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= gncma_StudioEquipmentMember                                            |
| Support equipment and systems                                                                       |                                                                                                                                                                                           |                                                                                                                                                                                           |
| **Property, Plant and Equipment [Line Items]**                                                      |                                                                                                                                                                                           |                                                                                                                                                                                           |
| Property, plant and equipment, gross                                                                | 270,629gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= gncma_SupportEquipmentAndSystemMember                                 | 245,301gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= gncma_SupportEquipmentAndSystemMember                                 |
| Transportation equipment                                                                            |                                                                                                                                                                                           |                                                                                                                                                                                           |
| **Property, Plant and Equipment [Line Items]**                                                      |                                                                                                                                                                                           |                                                                                                                                                                                           |
| Property, plant and equipment, gross                                                                | 15,667gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= us-gaap_TransportationEquipmentMember                                  | 13,619gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= us-gaap_TransportationEquipmentMember                                  |
| Customer premise equipment                                                                          |                                                                                                                                                                                           |                                                                                                                                                                                           |
| **Property, Plant and Equipment [Line Items]**                                                      |                                                                                                                                                                                           |                                                                                                                                                                                           |
| Property, plant and equipment, gross                                                                | 153,039gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= us-gaap_EquipmentLeasedToOtherPartyMember                             | 149,372gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= us-gaap_EquipmentLeasedToOtherPartyMember                             |
| Fiber optic cable systems                                                                           |                                                                                                                                                                                           |                                                                                                                                                                                           |
| **Property, Plant and Equipment [Line Items]**                                                      |                                                                                                                                                                                           |                                                                                                                                                                                           |
| Property, plant and equipment, gross                                                                | $ 305,200gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= gncma_FiberOpticCableSystemsMember                                  | $ 299,525gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= gncma_FiberOpticCableSystemsMember                                  |

| X                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The cumulative amount of amortization (related to property, plant and equipment, but not including land) that has been recognized in the income statement.
+ References
No definition available.
+ Details**Name:**gncma_AccumulatedAmortizationPropertyPlantandEquipment**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The cumulative amount of depreciation that has been recognized in the income statement.
+ References
No definition available.
+ Details**Name:**gncma_AccumulatedDepreciationPropertyPlantandEquipment**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tangible assets that are held by an entity and have been placed into service for use in the production or supply of goods and services, for rental to others, or for administrative purposes and that are expected to provide economic benefit for more than one year; net of accumulated depreciation. Examples include land, buildings, and production equipment. Excludes Construction in Progress.
+ References
No definition available.
+ Details**Name:**gncma_PropertyandEquipmentinServiceNet**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Property, Plant and Equipment, Gross, Excluding Construction in Progress
+ References
No definition available.
+ Details**Name:**gncma_PropertyPlantandEquipmentGrossExcludingConstructioninProgress**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before accumulated depreciation of leased physical assets used in the normal conduct of business to produce goods and services.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6455314&loc=d3e45014-112735
+ Details**Name:**us-gaap_CapitalLeasedAssetsGross**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_PropertyPlantAndEquipmentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_PropertyPlantAndEquipmentByTypeAxis=gncma_CableTransmissionEquipmentandDistributionFacilitiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_PropertyPlantAndEquipmentByTypeAxis=gncma_FiberOpticCableSystemsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_PropertyPlantAndEquipmentByTypeAxis=gncma_StudioEquipmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_PropertyPlantAndEquipmentByTypeAxis=gncma_SupportEquipmentAndSystemMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_PropertyPlantAndEquipmentByTypeAxis=gncma_TelephonyTransmissionEquipmentAndDistributionFacilitiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_EquipmentLeasedToOtherPartyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_LandAndBuildingMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_TransportationEquipmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |