| **Segments (Reportable segment revenues) (Details) - USD ($) $ in Thousands** | 3 Months Ended | 6 Months Ended |
|-------------------------------------------------------------------------------|----------------|----------------|
||
| **Segment Reporting Information [Line Items]**                                |           |           |   |   |
| Revenues                                                                      | $ 233,766      | $ 247,528      | $ 464,864 | $ 478,617 |
| Adjusted EBITDA                                                               | 79,040         | 88,001         | 157,146 | 163,304 |
| Operating Segments [Member]                                                   |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                |           |           |   |   |
| Revenues                                                                      | 233,766        | 247,528        | 464,864 | 478,617 |
| Adjusted EBITDA                                                               | 79,040         | 88,001         | 157,146 | 163,304 |
| Operating Segments [Member] | Wireless [Member]                               |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                |           |           |   |   |
| Revenues                                                                      | 53,875         | 67,940         | 105,337 | 127,144 |
| Adjusted EBITDA                                                               | 40,334         | 45,727         | 80,398 | 83,114 |
| Operating Segments [Member] | Wireline [Member]                               |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                |           |           |   |   |
| Revenues                                                                      | 179,891        | 179,588        | 359,527 | 351,473 |
| Adjusted EBITDA                                                               | $ 38,706       | $ 42,274       | $ 76,748 | $ 80,190 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Company's specifically defined non-GAAP measure which is defined as earnings plus cash received in excess of  revenue recognized for long-term fixed roaming arrangements and imputed interest on financed devices before: Net interest expense, Income taxes, Depreciation and amortization expense, Loss on extinguishment of debt, Software impairment charge, Derivative instrument unrealized income (loss), Share-based compensation expense, Accretion expense, Loss attributable to non-controlling interest resulting from NMTC transactions, Gains and impairment losses on equity and cost method investments, and Other non-cash adjustments.
+ 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=63488584&loc=d3e20235-122688
+ Details**Name:**us-gaap_SalesRevenueNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**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_SegmentReportingInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=gncma_WirelessSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=gncma_WirelineSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |