| **Commitments (Narratives) (Details) (USD $)** | 3 Months Ended | 9 Months Ended |               |
|------------------------------------------------|----------------|----------------|---------------|
||
| Revenues                                       | $ 178,494,000  | $ 177,703,000  | $ 526,505,000 | $ 510,569,000 |   |
| US Treasury Department Contribution            |           |           |          |   | 1,600,000 |
| Lifeline Program Support [Member]              |           |           |          |   |   |
| Revenues                                       | $ 3,300,000    | $ 4,200,000    | $ 11,300,000  | $ 12,900,000 |   |

| X                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of cash paid to settle a matter with the Federal Communication Commission and the Department of Justice.
+ References
No definition available.
+ Details**Name:**gncma_UsTreasuryDepartmentContribution**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| 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 SEC -Name Regulation S-X (SX) -Number 210 -Article 5 -Section 03 -Paragraph 1Reference 2: 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=6880815&loc=d3e20235-122688
+ Details**Name:**us-gaap_SalesRevenueNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |