| **Fair Value Measurements and Derivative Instruments Fair Value Measurements and Derivative Instruments (Derivative Financial Instruments) (Details) (USD $)** | 0 Months Ended                                                                                                                                                                                                  | 3 Months Ended                                                                                                                                                                                                |
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| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                              |                                                                                                                                                                                                            |                                                                                                                                                                                                          |
| Stock Appreciation Rights, Expiration Date                                                                                                                     | 8 years                                                                                                                                                                                                         |                                                                                                                                                                                                          |
| Unsecured Debt [Member] | Searchlight ALX, LP Promissory Note [Member]                                                                                         |                                                                                                                                                                                                            |                                                                                                                                                                                                          |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]**                                                             |                                                                                                                                                                                                            |                                                                                                                                                                                                          |
| Debt face amount                                                                                                                                               | 75,000,000.0us-gaap_DebtInstrumentFaceAmount/ us-gaap_DebtInstrumentAxis= gncma_SearchlightALXLPPromissoryNoteMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_UnsecuredDebtMember                                 |                                                                                                                                                                                                          |
| Stock Appreciation Rights (SARs) [Member]                                                                                                                      |                                                                                                                                                                                                            |                                                                                                                                                                                                          |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                              |                                                                                                                                                                                                            |                                                                                                                                                                                                          |
| Fair value measurement with unobservable inputs, beginning balance                                                                                             |                                                                                                                                                                                                            | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_ClassOfWarrantOrRightAxis= us-gaap_StockAppreciationRightsSARSMember                                       |
| Issuance                                                                                                                                                       |                                                                                                                                                                                                            | 21,660,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchasesSalesIssuancesSettlements/ us-gaap_ClassOfWarrantOrRightAxis= us-gaap_StockAppreciationRightsSARSMember |
| Fair value adjustment at end of period, included in Other Income (Expense)                                                                                     |                                                                                                                                                                                                            | 2,120,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_ClassOfWarrantOrRightAxis= us-gaap_StockAppreciationRightsSARSMember         |
| Fair value measurement with unobservable inputs, ending balance                                                                                                |                                                                                                                                                                                                            | $ 23,780,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_ClassOfWarrantOrRightAxis= us-gaap_StockAppreciationRightsSARSMember                            |
| Stock Appreciation Rights (SARs) [Member] | Common Stock - Class A [Member]                                                                                    |                                                                                                                                                                                                            |                                                                                                                                                                                                          |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]**                                                             |                                                                                                                                                                                                            |                                                                                                                                                                                                          |
| Stock Appreciation Rights                                                                                                                                      | 3,000,000us-gaap_ClassOfWarrantOrRightOutstanding/ us-gaap_ClassOfWarrantOrRightAxis= us-gaap_StockAppreciationRightsSARSMember/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember                  |                                                                                                                                                                                                          |
| Exercise price of stock appreciation rights (per share)                                                                                                        | 13.00us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1/ us-gaap_ClassOfWarrantOrRightAxis= us-gaap_StockAppreciationRightsSARSMember/ us-gaap_StatementClassOfStockAxis= us-gaap_CommonClassAMember |                                                                                                                                                                                                          |

| X                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Stock Appreciation Rights, Expiration Date
+ References
No definition available.
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| 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
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
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| X                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                   |
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of purchases, (sales), issuances and (settlements) of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph c -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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| X                                                                                                                                                                   |
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| X                                                                                                                                                |
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