| **Industry Segments Data (Reportable Segment) (Details) - USD ($) $ in Thousands** |        | 1 Months Ended | 3 Months Ended | 12 Months Ended |
|------------------------------------------------------------------------------------|--------|----------------|----------------|-----------------|
||
| **Segment Reporting Information [Line Items]**                                     |   |           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                           |   |           | $ 232,293      | $ 236,655       | $ 233,766 | $ 231,098 | $ 241,344 | $ 258,573 | $ 247,528 | $ 231,089 | $ 933,812 | $ 978,534 | $ 910,198 |
| Cost of Goods Sold                                                                 |   |           |           |            |   |   |   |   |   |   | 302,578 | 322,338 | 302,704 |
| Contribution                                                                       |   |           |           |            |   |   |   |   |   |   | 631,234 | 656,196 | 607,494 |
| Less SG&A                                                                      |   |           |           |            |   |   |   |   |   |   | (358,356) | (338,379) | (293,647) |
| Plus share-based compensation expense                                              |   |           |           |            |   |   |   |   |   |   | 11,043 | 10,902 | 8,392 |
| Plus imputed interest on financed devices                                          |   |           |           |            |   |   |   |   |   |   | 2,557 | 751 | 0 |
| Plus accretion expense                                                             |   |           |           |            |   |   |   |   |   |   | 1,229 | 1,121 | 1,249 |
| Other                                                                              |   |           |           |            |   |   |   |   |   |   | 337 | (240) | (372) |
| Adjusted EBITDA                                                                    |   |           |           |            |   |   |   |   |   |   | 288,044 | 330,351 | 323,116 |
| Capital expenditures                                                               | $ 746  | $ 49,794       |           |            |   |   |   |   |   |   | 194,478 | 176,235 | 176,109 |
| Goodwill                                                                           |   |           | 239,263        |            |   |   | 239,263 |   |   |   | 239,263 | 239,263 | 229,560 |
| Total assets                                                                       |   |           | 2,065,939      |            |   |   | 1,966,940 |   |   |   | 2,065,939 | 1,966,940 | 1,992,761 |
| Wholesale                                                                          |   |           |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information [Line Items]**                                     |   |           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                           |   |           |           |            |   |   |   |   |   |   | 208,109 | 267,676 | 269,977 |
| Consumer                                                                           |   |           |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information [Line Items]**                                     |   |           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                           |   |           |           |            |   |   |   |   |   |   | 340,460 | 351,196 | 288,014 |
| Business                                                                           |   |           |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information [Line Items]**                                     |   |           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                           |   |           |           |            |   |   |   |   |   |   | 385,243 | 359,662 | 352,207 |
| Wireless                                                                           |   |           |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information [Line Items]**                                     |   |           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                           |   |           |           |            |   |   |   |   |   |   | 208,109 | 267,676 | 269,977 |
| Cost of Goods Sold                                                                 |   |           |           |            |   |   |   |   |   |   | 62,487 | 70,899 | 90,920 |
| Contribution                                                                       |   |           |           |            |   |   |   |   |   |   | 145,622 | 196,777 | 179,057 |
| Less SG&A                                                                      |   |           |           |            |   |   |   |   |   |   | (16,439) | (18,137) | (21,631) |
| Plus share-based compensation expense                                              |   |           |           |            |   |   |   |   |   |   | 0 | 0 | 0 |
| Plus imputed interest on financed devices                                          |   |           |           |            |   |   |   |   |   |   | 0 | 0 |   |
| Plus accretion expense                                                             |   |           |           |            |   |   |   |   |   |   | 252 | 559 | 733 |
| Other                                                                              |   |           |           |            |   |   |   |   |   |   | 0 | 0 | 0 |
| Adjusted EBITDA                                                                    |   |           |           |            |   |   |   |   |   |   | 129,435 | 179,199 | 158,159 |
| Capital expenditures                                                               |   |           |           |            |   |   |   |   |   |   | 34,555 | 47,892 | 30,243 |
| Goodwill                                                                           |   |           | 164,312        |            |   |   | 164,312 |   |   |   | 164,312 | 164,312 | 164,312 |
| Total assets                                                                       |   |           | 601,796        |            |   |   | 594,250 |   |   |   | 601,796 | 594,250 | 625,417 |
| Wireless | Wholesale                                                               |   |           |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information [Line Items]**                                     |   |           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                           |   |           |           |            |   |   |   |   |   |   | 208,109 | 267,676 | 269,977 |
| Wireless | Consumer                                                                |   |           |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information [Line Items]**                                     |   |           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                           |   |           |           |            |   |   |   |   |   |   | 0 | 0 | 0 |
| Wireless | Business                                                                |   |           |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information [Line Items]**                                     |   |           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                           |   |           |           |            |   |   |   |   |   |   | 0 | 0 | 0 |
| Wireline                                                                           |   |           |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information [Line Items]**                                     |   |           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                           |   |           |           |            |   |   |   |   |   |   | 725,703 | 710,858 | 640,221 |
| Cost of Goods Sold                                                                 |   |           |           |            |   |   |   |   |   |   | 240,091 | 251,439 | 211,784 |
| Contribution                                                                       |   |           |           |            |   |   |   |   |   |   | 485,612 | 459,419 | 428,437 |
| Less SG&A                                                                      |   |           |           |            |   |   |   |   |   |   | (341,917) | (320,242) | (272,016) |
| Plus share-based compensation expense                                              |   |           |           |            |   |   |   |   |   |   | 11,043 | 10,902 | 8,392 |
| Plus imputed interest on financed devices                                          |   |           |           |            |   |   |   |   |   |   | 2,557 | 751 |   |
| Plus accretion expense                                                             |   |           |           |            |   |   |   |   |   |   | 977 | 562 | 516 |
| Other                                                                              |   |           |           |            |   |   |   |   |   |   | 337 | (240) | (372) |
| Adjusted EBITDA                                                                    |   |           |           |            |   |   |   |   |   |   | 158,609 | 151,152 | 164,957 |
| Capital expenditures                                                               |   |           |           |            |   |   |   |   |   |   | 159,923 | 128,343 | 145,866 |
| Goodwill                                                                           |   |           | 74,951         |            |   |   | 74,951 |   |   |   | 74,951 | 74,951 | 65,248 |
| Total assets                                                                       |   |           | $ 1,464,143    |            |   |   | $ 1,372,690 |   |   |   | 1,464,143 | 1,372,690 | 1,367,344 |
| Wireline | Wholesale                                                               |   |           |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information [Line Items]**                                     |   |           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                           |   |           |           |            |   |   |   |   |   |   | 0 | 0 | 0 |
| Wireline | Consumer                                                                |   |           |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information [Line Items]**                                     |   |           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                           |   |           |           |            |   |   |   |   |   |   | 340,460 | 351,196 | 288,014 |
| Wireline | Business                                                                |   |           |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information [Line Items]**                                     |   |           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                           |   |           |           |            |   |   |   |   |   |   | $ 385,243 | $ 359,662 | $ 352,207 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Earnings plus 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,Gain recorded for adjusting to fair value assets that were included as consideration paid to acquire a fiber system, andOther non-cash adjustments.
+ References
No definition available.
+ Details**Name:**gncma_AdjustedEBITDA**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Imputed interest income from receivables with financing component.
+ References
No definition available.
+ Details**Name:**gncma_InterestIncomeReceivables**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848
+ Details**Name:**us-gaap_AssetRetirementObligationAccretionExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(12)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(11)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878
+ Details**Name:**us-gaap_Assets**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate cost of goods produced and sold during the reporting period, excluding depreciation, depletion, and amortization.
+ References
No definition available.
+ Details**Name:**us-gaap_CostOfGoodsSoldExcludingDepreciationDepletionAndAmortization**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=64834752&loc=SL49117168-202975
+ Details**Name:**us-gaap_Goodwill**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity.
+ 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 210.5-03.1,2) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688
+ Details**Name:**us-gaap_GrossProfit**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The net amount of other operating income and expenses, the components of which are not separately disclosed on the income statement, from items that are associated with the entity's normal revenue producing operations.
+ References
No definition available.
+ Details**Name:**us-gaap_OtherOperatingIncomeExpenseNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of acquisition of long-lived, physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.
+ References
No definition available.
+ Details**Name:**us-gaap_PropertyPlantAndEquipmentAdditions**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate total costs related to selling a firm's product and services, as well as all other general and administrative expenses. Direct selling expenses (for example, credit, warranty, and advertising) are expenses that can be directly linked to the sale of specific products. Indirect selling expenses are expenses that cannot be directly linked to the sale of specific products, for example telephone expenses, Internet, and postal charges. General and administrative expenses include salaries of non-sales personnel, rent, utilities, communication, etc.
+ 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 210.5-03.4) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 330 -SubTopic 10 -Section 30 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=68048583&loc=d3e3636-108311
+ Details**Name:**us-gaap_SellingGeneralAndAdministrativeExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585
+ Details**Name:**us-gaap_ShareBasedCompensation**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_SubsegmentsAxis=gncma_WholesaleMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_SubsegmentsAxis=gncma_ConsumerMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_SubsegmentsAxis=gncma_BusinessServicesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| 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:** |