Annual report pursuant to Section 13 and 15(d)

Industry Segments Data (Narrative) (Details)

v2.4.1.9
Industry Segments Data (Narrative) (Details) (USD $)
12 Months Ended
Dec. 31, 2014
segment
Dec. 31, 2013
Dec. 31, 2012
Segment Reporting Information [Line Items]      
Number of reportable segments 2us-gaap_NumberOfReportableSegments    
Percentage of long-lived assets not in US 82.00%gncma_SegmentReportingInformationPercentageofLongLivedAssetsNotLocatedinCountryofOrigin    
Revenues $ 910,198,000us-gaap_Revenues $ 811,648,000us-gaap_Revenues $ 710,181,000us-gaap_Revenues
Wireless      
Segment Reporting Information [Line Items]      
Equipment subsidy 17,300,000gncma_EquipmentSubsidy
/ us-gaap_StatementBusinessSegmentsAxis
= gncma_WirelessSegmentMember
12,200,000gncma_EquipmentSubsidy
/ us-gaap_StatementBusinessSegmentsAxis
= gncma_WirelessSegmentMember
23,200,000gncma_EquipmentSubsidy
/ us-gaap_StatementBusinessSegmentsAxis
= gncma_WirelessSegmentMember
Revenues 269,977,000us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= gncma_WirelessSegmentMember
197,218,000us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= gncma_WirelessSegmentMember
124,745,000us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= gncma_WirelessSegmentMember
Customer Concentration Risk      
Segment Reporting Information [Line Items]      
Concentration Risk, Percentage 6.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
   
Customer Concentration Risk | Sales Revenue, Net | Wireless and Wireline      
Segment Reporting Information [Line Items]      
Revenues $ 108,300,000us-gaap_Revenues
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_StatementBusinessSegmentsAxis
= gncma_WirelessandWirelineMember
   
Concentration Risk, Percentage 12.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_StatementBusinessSegmentsAxis
= gncma_WirelessandWirelineMember