| **Net Property and Equipment (Narrative) (Details) - USD ($) $ in Thousands** | 12 Months Ended |
|-------------------------------------------------------------------------------|-----------------|
||
| **Property, Plant and Equipment [Line Items]**                                |            |   |   |
| Cash payment                                                                  | $ 19,700        | $ 0 | $ 0 |
| Fair Market Value of IRU Capacity                                             | 14,000          |   |   |
| Other Income (Expense) [Member]                                               |            |   |   |
| **Property, Plant and Equipment [Line Items]**                                |            |   |   |
| Gain from adjustment to fair value                                            | $ 3,100         |   |   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Noncash or Part Noncash consideration provider to buyer of a business or asset. Noncash is defined as transactions during a period that affect recognized assets or liabilities, but do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.
+ References
No definition available.
+ Details**Name:**gncma_NoncashorPartNoncashAcquisitionNoncashConsideration**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The cash outflow to acquire fiber assets.
+ References
No definition available.
+ Details**Name:**gncma_PaymentstoAcquireFiberAssets**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of addition (reduction) to the amount at which an asset could be incurred (settled) in a current transaction between willing parties.
+ References
No definition available.
+ Details**Name:**us-gaap_AssetsFairValueAdjustment**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_PropertyPlantAndEquipmentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_IncomeStatementLocationAxis=us-gaap_OtherNonoperatingIncomeExpenseMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |