| **Segments (Reportable segment revenues) (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended | 6 Months Ended |
|------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Segment Reporting Information [Line Items]**                                                       |           |           |   |   |
| Revenues                                                                                             | $ 224,399      | $ 189,661      | $ 440,682 | $ 375,877 |
| Reportable segment Adjusted EBITDA                                                                   | 84,471         | 62,139         | 159,268 | 120,788 |
| Wireless [Member]                                                                                    |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                       |           |           |   |   |
| Revenues                                                                                             | 69,397         | 35,559         | 131,914 | 69,396 |
| Reportable segment Adjusted EBITDA                                                                   | 40,174         | 14,273         | 78,196 | 29,462 |
| Wireline [Member]                                                                                    |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                       |           |           |   |   |
| Revenues                                                                                             | 155,002        | 154,102        | 308,768 | 306,481 |
| Reportable segment Adjusted EBITDA                                                                   | $ 44,297       | $ 47,866       | $ 81,072 | $ 91,326 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents Company's specificly defined non-GAAP measure - the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses from ongoing operations, after income or loss from equity method investments, but before income taxes, extraordinary items, accretion expense, non-cash contribution expense and noncontrolling interest.
+ References
No definition available.
+ Details**Name:**gncma_IncomeLossfromContinuingOperationsbeforeEquityMethodInvestmentsIncomeTaxesAdjusted**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts.
+ 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 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Article 5
+ Details**Name:**us-gaap_SalesRevenueNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_SegmentReportingInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |