Quarterly report pursuant to Section 13 or 15(d)

Business and Summary of Significant Accounting Principles (Narratives) (Details)

v2.4.1.9
Business and Summary of Significant Accounting Principles (Narratives) (Details) (USD $)
Share data in Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
entity
Mar. 31, 2014
Dec. 31, 2014
Feb. 02, 2015
Business        
Year founded 1979      
Principles of Consolidation        
Number of VIEs 4us-gaap_VariableInterestEntityNumberOfEntitiesWithUnavailabilityOfInformation      
Common Stock        
Stock repurchased during period, value $ 16,993,000us-gaap_StockRepurchasedAndRetiredDuringPeriodValue $ 529,000us-gaap_StockRepurchasedAndRetiredDuringPeriodValue    
Accounts Receivable and Allowance for Doubtful Receivables        
Period past due for write-off of trade accounts receivable 120 days      
Revenue Recognition        
Revenues 231,089,000us-gaap_SalesRevenueNet 216,283,000us-gaap_SalesRevenueNet    
Receivables 198,721,000us-gaap_AccountsReceivableGrossCurrent   212,441,000us-gaap_AccountsReceivableGrossCurrent  
Total High Cost Support Program [Member]        
Revenue Recognition        
Revenues 17,300,000us-gaap_SalesRevenueNet
/ gncma_TypeOfProgramAxis
= gncma_HighCostSupportProgramMember
16,500,000us-gaap_SalesRevenueNet
/ gncma_TypeOfProgramAxis
= gncma_HighCostSupportProgramMember
   
Receivables 47,100,000us-gaap_AccountsReceivableGrossCurrent
/ gncma_TypeOfProgramAxis
= gncma_HighCostSupportProgramMember
     
Common Stock - Class A [Member]        
Common Stock        
Stock repurchased during period, shares 1,091us-gaap_StockRepurchasedAndRetiredDuringPeriodShares
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
26us-gaap_StockRepurchasedAndRetiredDuringPeriodShares
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
   
Stock repurchased during period, value 16,878,000us-gaap_StockRepurchasedAndRetiredDuringPeriodValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
529,000us-gaap_StockRepurchasedAndRetiredDuringPeriodValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
   
Stock Buyback Program [Member]        
Common Stock        
Authorized amount, repurchase of stock 5,000,000us-gaap_StockRepurchaseProgramAuthorizedAmount1
/ us-gaap_ShareRepurchaseProgramAxis
= gncma_StockBuybackProgramMember
     
Stock repurchase program, remaining value authorized to be repurchased 111,400,000us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1
/ us-gaap_ShareRepurchaseProgramAxis
= gncma_StockBuybackProgramMember
     
Stock Buyback Program [Member] | Common Stock - Class A [Member]        
Common Stock        
Stock repurchased during period, shares 1,100us-gaap_StockRepurchasedAndRetiredDuringPeriodShares
/ us-gaap_ShareRepurchaseProgramAxis
= gncma_StockBuybackProgramMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
0us-gaap_StockRepurchasedAndRetiredDuringPeriodShares
/ us-gaap_ShareRepurchaseProgramAxis
= gncma_StockBuybackProgramMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
   
Stock repurchased during period, value $ 16,100,000us-gaap_StockRepurchasedAndRetiredDuringPeriodValue
/ us-gaap_ShareRepurchaseProgramAxis
= gncma_StockBuybackProgramMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
     
ACS Wireless [Member]        
Principles of Consolidation        
Voting interests acquired       33.33%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired
/ us-gaap_BusinessAcquisitionAxis
= gncma_ACSWirelessMember
Interests acquired from noncontrolling interest       66.67%gncma_BusinessAcquisitionPercentageofVotingInterestsAcquiredfromNoncontrollingInterest
/ us-gaap_BusinessAcquisitionAxis
= gncma_ACSWirelessMember