| **Industry Segments Data (Narratives) (Details) $ in Thousands** | 3 Months Ended | 12 Months Ended |
|------------------------------------------------------------------|----------------|-----------------|
||
| **Segment Reporting Information [Line Items]**                   |           |            |   |   |   |   |   |   |   |   |   |
| Number of reportable segments | segment                          |           |            |   |   |   |   |   |   | 2 |   |   |
| Percentage of long-lived assets not in US                        |           |            |   |   |   |   |   |   | 82.00% |   |   |
| Number of Major Customers                                        |           |            |   |   |   |   |   |   | 0 |   |   |
| Revenues                                                         | $ 232,293      | $ 236,655       | $ 233,766 | $ 231,098 | $ 241,344 | $ 258,573 | $ 247,528 | $ 231,089 | $ 933,812 | $ 978,534 | $ 910,198 |
| Customer Concentration Risk | Sales Revenue, Net                 |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information [Line Items]**                   |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                         |           |            |   |   |   |   |   |   |   | $ 130,800 | $ 108,300 |
| Concentration Risk, Percentage                                   |           |            |   |   |   |   |   |   |   | 13.00% | 12.00% |
| Wireless                                                         |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information [Line Items]**                   |           |            |   |   |   |   |   |   |   |   |   |
| Equipment subsidy                                                |           |            |   |   |   |   |   |   | 0 | $ 7,700 | $ 17,300 |
| Revenues                                                         |           |            |   |   |   |   |   |   | $ 208,109 | $ 267,676 | $ 269,977 |

| X                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Equipment Subsidy
+ References
No definition available.
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| X                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Major Customers
+ References
No definition available.
+ Details**Name:**gncma_NumberofMajorCustomers**Namespace Prefix:**gncma_**Data Type:**xbrli:integerItemType**Balance Type:**na**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
For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=64851502&loc=d3e6351-108592Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13531-108611Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=64851502&loc=d3e6404-108592Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13537-108611
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| 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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
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| 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_ConcentrationRiskByBenchmarkAxis=us-gaap_SalesRevenueNetMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
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