| **Subsequent Events (Details) (Subsequent Event [Member], USD $)**                                                                                       | 0 Months Ended                                                                                                                                                                                                                                                                                                                                                         |
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| ACS Wireless [Member]                                                                                                                                    |                                                                                                                                                                                                                                                                                                                                                                   |
| **Subsequent Event [Line Items]**                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                   |
| Cash consideration paid                                                                                                                                  | $ 293,200,000us-gaap_PaymentsToAcquireBusinessesGross/ us-gaap_BusinessAcquisitionAxis= gncma_ACSWirelessMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember                                                                                                                                                                                        |
| Medium-term Notes [Member] | Credit Agricole Fourth Amended and Restated Credit and Guarantee Agreement [Member]                                         |                                                                                                                                                                                                                                                                                                                                                                   |
| **Subsequent Event [Line Items]**                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                   |
| Debt Instrument, face Amount                                                                                                                             | 275,000,000.0us-gaap_DebtInstrumentFaceAmount/ us-gaap_DebtInstrumentAxis= gncma_CreditAgricoleFourthAmendedandRestatedCreditandGuaranteeAgreementMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_MediumTermNotesMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember                                                                                  |
| Medium-term Notes [Member] | Credit Agricole Fourth Amended and Restated Credit and Guarantee Agreement [Member] | London Interbank Offered Rate (LIBOR) |                                                                                                                                                                                                                                                                                                                                                                   |
| **Subsequent Event [Line Items]**                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                   |
| Basis spread on variable rate                                                                                                                            | 3.75%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1/ us-gaap_DebtInstrumentAxis= gncma_CreditAgricoleFourthAmendedandRestatedCreditandGuaranteeAgreementMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_MediumTermNotesMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember/ us-gaap_VariableRateAxis= us-gaap_LondonInterbankOfferedRateLIBORMember |
| Variable rate floor                                                                                                                                      | 1.00%gncma_DebtInstrumentVariableRateFloor/ us-gaap_DebtInstrumentAxis= gncma_CreditAgricoleFourthAmendedandRestatedCreditandGuaranteeAgreementMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_MediumTermNotesMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember/ us-gaap_VariableRateAxis= us-gaap_LondonInterbankOfferedRateLIBORMember            |
| Unsecured Debt [Member] | Searchlight ALX, LP Promissory Note [Member]                                                                                   |                                                                                                                                                                                                                                                                                                                                                                   |
| **Subsequent Event [Line Items]**                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                   |
| Debt Instrument, face Amount                                                                                                                             | $ 75,000,000.0us-gaap_DebtInstrumentFaceAmount/ us-gaap_DebtInstrumentAxis= gncma_SearchlightALXLPPromissoryNoteMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_UnsecuredDebtMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember                                                                                                                      |
| Stated percentage                                                                                                                                        | 7.50%us-gaap_DebtInstrumentInterestRateStatedPercentage/ us-gaap_DebtInstrumentAxis= gncma_SearchlightALXLPPromissoryNoteMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_UnsecuredDebtMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember                                                                                                             |
| Class A Common Stock | Stock Appreciation Rights (SARs) [Member]                                                                                         |                                                                                                                                                                                                                                                                                                                                                                   |
| **Subsequent Event [Line Items]**                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                   |
| Rights outstanding                                                                                                                                       | 3,000,000us-gaap_ClassOfWarrantOrRightOutstanding/ us-gaap_ClassOfWarrantOrRightAxis= us-gaap_StockAppreciationRightsSARSMember/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember                                                                                                         |
| Exercise price of rights                                                                                                                                 | $ 13.00us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1/ us-gaap_ClassOfWarrantOrRightAxis= us-gaap_StockAppreciationRightsSARSMember/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember                                                                                      |

| 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
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
+ Details**Name:**us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1**Namespace Prefix:**us-gaap_**Data Type:**num:perShareItemType**Balance Type:**na**Period Type:**instant |
| 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
+ Details**Name:**us-gaap_ClassOfWarrantOrRightOutstanding**Namespace Prefix:**us-gaap_**Data Type:**xbrli:sharesItemType**Balance Type:**na**Period Type:**instant |
| 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
+ Details**Name:**us-gaap_DebtInstrumentInterestRateStatedPercentage**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585
+ Details**Name:**us-gaap_PaymentsToAcquireBusinessesGross**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_SubsequentEventLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LongtermDebtTypeAxis=us-gaap_MediumTermNotesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LongtermDebtTypeAxis=us-gaap_UnsecuredDebtMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |