| **Business and Summary of Significant Accounting Principles (Narratives) (Details) (USD $)Share data in Thousands, unless otherwise specified** | 3 Months Ended                                                                                                                                                                                      |                                                                                                                                                                                           |                                                   |
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||
| **Business**                                                                                                                                    |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| Year founded                                                                                                                                    | 1979                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| **Principles of Consolidation**                                                                                                                 |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| Number of VIEs                                                                                                                                  | 4us-gaap_VariableInterestEntityNumberOfEntitiesWithUnavailabilityOfInformation                                                                                                                      |                                                                                                                                                                                      |                                              |   |
| **Common Stock**                                                                                                                                |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| Stock repurchased during period, value                                                                                                          | $ 16,993,000us-gaap_StockRepurchasedAndRetiredDuringPeriodValue                                                                                                                                     | $ 529,000us-gaap_StockRepurchasedAndRetiredDuringPeriodValue                                                                                                                              |                                              |   |
| **Accounts Receivable and Allowance for Doubtful Receivables**                                                                                  |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| Period past due for write-off of trade accounts receivable                                                                                      | 120 days                                                                                                                                                                                            |                                                                                                                                                                                      |                                              |   |
| **Revenue Recognition**                                                                                                                         |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| Revenues                                                                                                                                        | 231,089,000us-gaap_SalesRevenueNet                                                                                                                                                                  | 216,283,000us-gaap_SalesRevenueNet                                                                                                                                                        |                                              |   |
| Receivables                                                                                                                                     | 198,721,000us-gaap_AccountsReceivableGrossCurrent                                                                                                                                                   |                                                                                                                                                                                      | 212,441,000us-gaap_AccountsReceivableGrossCurrent |   |
| Total High Cost Support Program [Member]                                                                                                        |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| **Revenue Recognition**                                                                                                                         |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| Revenues                                                                                                                                        | 17,300,000us-gaap_SalesRevenueNet/ gncma_TypeOfProgramAxis= gncma_HighCostSupportProgramMember                                                                                                      | 16,500,000us-gaap_SalesRevenueNet/ gncma_TypeOfProgramAxis= gncma_HighCostSupportProgramMember                                                                                            |                                              |   |
| Receivables                                                                                                                                     | 47,100,000us-gaap_AccountsReceivableGrossCurrent/ gncma_TypeOfProgramAxis= gncma_HighCostSupportProgramMember                                                                                       |                                                                                                                                                                                      |                                              |   |
| Common Stock - Class A [Member]                                                                                                                 |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| **Common Stock**                                                                                                                                |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| Stock repurchased during period, shares                                                                                                         | 1,091us-gaap_StockRepurchasedAndRetiredDuringPeriodShares/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember                                                                            | 26us-gaap_StockRepurchasedAndRetiredDuringPeriodShares/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember                                                                     |                                              |   |
| Stock repurchased during period, value                                                                                                          | 16,878,000us-gaap_StockRepurchasedAndRetiredDuringPeriodValue/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember                                                                        | 529,000us-gaap_StockRepurchasedAndRetiredDuringPeriodValue/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember                                                                 |                                              |   |
| Stock Buyback Program [Member]                                                                                                                  |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| **Common Stock**                                                                                                                                |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| Authorized amount, repurchase of stock                                                                                                          | 5,000,000us-gaap_StockRepurchaseProgramAuthorizedAmount1/ us-gaap_ShareRepurchaseProgramAxis= gncma_StockBuybackProgramMember                                                                       |                                                                                                                                                                                      |                                              |   |
| Stock repurchase program, remaining value authorized to be repurchased                                                                          | 111,400,000us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1/ us-gaap_ShareRepurchaseProgramAxis= gncma_StockBuybackProgramMember                                                  |                                                                                                                                                                                      |                                              |   |
| Stock Buyback Program [Member] | Common Stock - Class A [Member]                                                                                |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| **Common Stock**                                                                                                                                |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| Stock repurchased during period, shares                                                                                                         | 1,100us-gaap_StockRepurchasedAndRetiredDuringPeriodShares/ us-gaap_ShareRepurchaseProgramAxis= gncma_StockBuybackProgramMember/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember       | 0us-gaap_StockRepurchasedAndRetiredDuringPeriodShares/ us-gaap_ShareRepurchaseProgramAxis= gncma_StockBuybackProgramMember/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember |                                              |   |
| Stock repurchased during period, value                                                                                                          | $ 16,100,000us-gaap_StockRepurchasedAndRetiredDuringPeriodValue/ us-gaap_ShareRepurchaseProgramAxis= gncma_StockBuybackProgramMember/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember |                                                                                                                                                                                      |                                              |   |
| ACS Wireless [Member]                                                                                                                           |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| **Principles of Consolidation**                                                                                                                 |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              |   |
| Voting interests acquired                                                                                                                       |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              | 33.33%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired/ us-gaap_BusinessAcquisitionAxis= gncma_ACSWirelessMember |
| Interests acquired from noncontrolling interest                                                                                                 |                                                                                                                                                                                                |                                                                                                                                                                                      |                                              | 66.67%gncma_BusinessAcquisitionPercentageofVotingInterestsAcquiredfromNoncontrollingInterest/ us-gaap_BusinessAcquisitionAxis= gncma_ACSWirelessMember |

| X                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Business Acquisition, Percentage of Voting Interests Acquired from Noncontrolling Interest
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amounts due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer) for goods or services (including trade receivables) that have been delivered or sold in the normal course of business.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of voting equity interests acquired at the acquisition date in the business combination.
+ References
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| X                                                                                                                                                  |
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| 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
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of shares that have been repurchased and retired during the period.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Equity impact of the value of stock that has been repurchased and retired during the period. The excess of the purchase price over par value can be charged against retained earnings (once the excess is fully allocated to additional paid in capital).
+ References
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| X                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of stock repurchase plan authorized.
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| - Definition
Amount remaining of a stock repurchase plan authorized.
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No definition available.
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| - Definition
Threshold period past due for trade accounts receivable to write off as uncollectible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Variable Interest Entities (VIE) or potential VIEs for which the necessary information is unable to be obtained to: (1) determine whether the legal entity is a VIE, (2) determine whether it (the reporting entity) is the VIE's primary beneficiary, or (3) perform the accounting required to consolidate the VIE for which it is determined that it (the reporting entity) is the primary beneficiary.
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| X                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Year the entity was founded, in CCYY format.
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