| **Industry Segments Data (Narrative) (Details) (USD $)In Millions, unless otherwise specified** | 12 Months Ended | 7 Months Ended | 12 Months Ended |
|-------------------------------------------------------------------------------------------------|-----------------|----------------|-----------------|
||
| **Segment Reporting Information [Line Items]**                                                  |            |           |            |   |
| Number of reportable segments                                                                   | 2               |           |            |   |
| Equipment subsidy                                                                               |            | $ 13.0         | $ 23.2          | $ 16.5 |
| Percentage of long-lived assets not in US                                                       | 82.00%          |           |            |   |

| X                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Equipment Subsidy
+ References
No definition available.
+ Details**Name:**gncma_EquipmentSubsidy**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Segment Reporting Information, Percentage of Long-Lived Assets Not Located in Country of Origin
+ References
No definition available.
+ Details**Name:**gncma_SegmentReportingInformationPercentageofLongLivedAssetsNotLocatedinCountryofOrigin**Namespace Prefix:**gncma_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.
+ References
No definition available.
+ Details**Name:**us-gaap_NumberOfReportableSegments**Namespace Prefix:**us-gaap_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_SegmentReportingInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |