| **Commitments and Contingencies (Narrative) (Details) (USD $)In Millions, unless otherwise specified** | 9 Months Ended |
|--------------------------------------------------------------------------------------------------------|----------------|
||
| **Business Acquisition [Line Items]**                                                                  |           |
| Construction costs                                                                                     | $ 20.7         |
| ACS [Member]                                                                                           |           |
| **Business Acquisition [Line Items]**                                                                  |           |
| Preferential adjustment maximum                                                                        | $ 21.8         |

| X                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Business Combination, Preferential Adjustment Maximum
+ References
No definition available.
+ Details**Name:**gncma_BusinessCombinationPreferentialAdjustmentMaximum**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_BusinessAcquisitionLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate costs related to construction and development services during the reporting period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.2) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688
+ Details**Name:**us-gaap_ConstructionAndDevelopmentCosts**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |