Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements and Derivative Instruments Fair Value Measurements and Derivative Instruments (Derivative Financial Instruments) (Details)

v2.4.1.9
Fair Value Measurements and Derivative Instruments Fair Value Measurements and Derivative Instruments (Derivative Financial Instruments) (Details) (USD $)
 0 Months Ended 3 Months Ended
Feb. 02, 2015
Mar. 31, 2015
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