| **Long-Term Debt (Details) (USD $)**                                                                                  | 3 Months Ended                                 | 0 Months Ended                         |
|-----------------------------------------------------------------------------------------------------------------------|------------------------------------------------|----------------------------------------|
||
| **Debt Instrument [Line Items]**                                                                                      |                                           |                                   |   |
| Loan fees and other expenses                                                                                          | $ 5,853,000us-gaap_PaymentsOfDebtIssuanceCosts | $ 0us-gaap_PaymentsOfDebtIssuanceCosts |   |
| Medium-term Notes [Member] | Amended Senior Credit Facility [Member]                                                  |                                           |                                   |   |
| **Debt Instrument [Line Items]**                                                                                      |                                           |                                   |   |
| Debt face amount                                                                                                      |                                           |                                   | 275,000,000.0us-gaap_DebtInstrumentFaceAmount/ us-gaap_DebtInstrumentAxis= gncma_AmendedSeniorCreditFacilityMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_MediumTermNotesMember |
| Percentage of principal payable                                                                                       |                                           |                                   | 0.25%gncma_DebtInstrumentPeriodicPaymentPrincipalPercentage/ us-gaap_DebtInstrumentAxis= gncma_AmendedSeniorCreditFacilityMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_MediumTermNotesMember |
| Loan fees and other expenses                                                                                          |                                           |                                   | 5,900,000us-gaap_PaymentsOfDebtIssuanceCosts/ us-gaap_DebtInstrumentAxis= gncma_AmendedSeniorCreditFacilityMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_MediumTermNotesMember |
| Medium-term Notes [Member] | Amended Senior Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] |                                           |                                   |   |
| **Debt Instrument [Line Items]**                                                                                      |                                           |                                   |   |
| Spread on variable rate                                                                                               |                                           |                                   | 3.75%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1/ us-gaap_DebtInstrumentAxis= gncma_AmendedSeniorCreditFacilityMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_MediumTermNotesMember/ us-gaap_VariableRateAxis= us-gaap_LondonInterbankOfferedRateLIBORMember |
| Interest rate floor                                                                                                   |                                           |                                   | 1.00%gncma_DebtInstrumentVariableRateFloor/ us-gaap_DebtInstrumentAxis= gncma_AmendedSeniorCreditFacilityMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_MediumTermNotesMember/ us-gaap_VariableRateAxis= us-gaap_LondonInterbankOfferedRateLIBORMember |
| Unsecured Debt [Member] | Searchlight ALX, LP Promissory Note [Member]                                                |                                           |                                   |   |
| **Debt Instrument [Line Items]**                                                                                      |                                           |                                   |   |
| Debt face amount                                                                                                      |                                           |                                   | 75,000,000.0us-gaap_DebtInstrumentFaceAmount/ us-gaap_DebtInstrumentAxis= gncma_SearchlightALXLPPromissoryNoteMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_UnsecuredDebtMember |
| Debt stated percentage                                                                                                |                                           |                                   | 7.50%us-gaap_DebtInstrumentInterestRateStatedPercentage/ us-gaap_DebtInstrumentAxis= gncma_SearchlightALXLPPromissoryNoteMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_UnsecuredDebtMember |
| Unamortized discount                                                                                                  |                                           |                                   | $ 21,700,000.0us-gaap_DebtInstrumentUnamortizedDiscount/ us-gaap_DebtInstrumentAxis= gncma_SearchlightALXLPPromissoryNoteMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_UnsecuredDebtMember |
| Common Stock - Class A [Member] | Stock Appreciation Rights (SARs) [Member]                                           |                                           |                                   |   |
| **Debt Instrument [Line Items]**                                                                                      |                                           |                                   |   |
| Exercise price of stock appreciation rights (per share)                                                               |                                           |                                   | $ 13.00us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1/ us-gaap_ClassOfWarrantOrRightAxis= us-gaap_StockAppreciationRightsSARSMember/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember |
| Stock Appreciation Rights                                                                                             |                                           |                                   | 3,000,000us-gaap_ClassOfWarrantOrRightOutstanding/ us-gaap_ClassOfWarrantOrRightAxis= us-gaap_StockAppreciationRightsSARSMember/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember |
| Stock Appreciation Rights issued for interest capitalization                                                          |                                           |                                   | 4.00%gncma_PercentofRightPayableperDollarofCapitalizedInterest/ us-gaap_ClassOfWarrantOrRightAxis= us-gaap_StockAppreciationRightsSARSMember/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember |

| X                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Debt Instrument, Periodic Payment, Principal, Percentage
+ References
No definition available.
+ Details**Name:**gncma_DebtInstrumentPeriodicPaymentPrincipalPercentage**Namespace Prefix:**gncma_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Debt Instrument, Variable Rate Floor
+ References
No definition available.
+ Details**Name:**gncma_DebtInstrumentVariableRateFloor**Namespace Prefix:**gncma_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percent of a right payable per dollar of capitalized interest, payable if interest on debt is capitalized instead of paid when due.
+ References
No definition available.
+ Details**Name:**gncma_PercentofRightPayableperDollarofCapitalizedInterest**Namespace Prefix:**gncma_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Exercise price per share or per unit of warrants or rights outstanding.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(i)(4)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of warrants or rights outstanding.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(i)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph i -Article 4
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| X                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage points added to the reference rate to compute the variable rate on the debt instrument.
+ References
No definition available.
+ Details**Name:**us-gaap_DebtInstrumentBasisSpreadOnVariableRate1**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Face (par) amount of debt instrument at time of issuance.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400
+ Details**Name:**us-gaap_DebtInstrumentFaceAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Contractual interest rate for funds borrowed, under the debt agreement.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DebtInstrumentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400
+ Details**Name:**us-gaap_DebtInstrumentUnamortizedDiscount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The cash outflow paid to third parties in connection with debt origination, which will be amortized over the remaining maturity period of the associated long-term debt.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585
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| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LongtermDebtTypeAxis=us-gaap_UnsecuredDebtMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
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