| **Long Term Debt (Schedule of Long Term Debt Applicable Margin) (Details) (London Interbank Offered Rate (LIBOR))** | 12 Months Ended                                                                                                                                                                                                                              |
|---------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| Greater than or equal to 5.5                                                                                        |                                                                                                                                                                                                                                         |
| **Debt Instrument [Line Items]**                                                                                    |                                                                                                                                                                                                                                         |
| Basis spread on variable rate                                                                                       | 3.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1/ gncma_InterestRateLeverageRatioAxis= gncma_LeverageRatioRangeOneMember/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis= us-gaap_LondonInterbankOfferedRateLIBORMember   |
| Greater than or equal to 5.0 but less than or equal to 5.5                                                          |                                                                                                                                                                                                                                         |
| **Debt Instrument [Line Items]**                                                                                    |                                                                                                                                                                                                                                         |
| Basis spread on variable rate                                                                                       | 2.75%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1/ gncma_InterestRateLeverageRatioAxis= gncma_LeverageRatioRangeTwoMember/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis= us-gaap_LondonInterbankOfferedRateLIBORMember   |
| Greater than or equal to 4.5 but less than or equal to 5.0                                                          |                                                                                                                                                                                                                                         |
| **Debt Instrument [Line Items]**                                                                                    |                                                                                                                                                                                                                                         |
| Basis spread on variable rate                                                                                       | 2.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1/ gncma_InterestRateLeverageRatioAxis= gncma_LeverageRatioRangeThreeMember/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis= us-gaap_LondonInterbankOfferedRateLIBORMember |
| Greater than or equal to 4.0 but less than or equal to 4.5                                                          |                                                                                                                                                                                                                                         |
| **Debt Instrument [Line Items]**                                                                                    |                                                                                                                                                                                                                                         |
| Basis spread on variable rate                                                                                       | 2.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1/ gncma_InterestRateLeverageRatioAxis= gncma_LeverageRatioRangeFourMember/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis= us-gaap_LondonInterbankOfferedRateLIBORMember  |
| Less than 4.0                                                                                                       |                                                                                                                                                                                                                                         |
| **Debt Instrument [Line Items]**                                                                                    |                                                                                                                                                                                                                                         |
| Basis spread on variable rate                                                                                       | 2.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1/ gncma_InterestRateLeverageRatioAxis= gncma_LeverageRatioRangeFiveMember/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis= us-gaap_LondonInterbankOfferedRateLIBORMember  |

| X                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage points added to the reference rate to compute the variable rate on the debt instrument.
+ References
No definition available.
+ Details**Name:**us-gaap_DebtInstrumentBasisSpreadOnVariableRate1**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DebtInstrumentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |