Annual report pursuant to Section 13 and 15(d)

Allowance for Doubtful Receivables (Allowance for Doubtful Receivables Rollforward) (Details)

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Allowance for Doubtful Receivables (Allowance for Doubtful Receivables Rollforward) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Balance at beginning of year $ 2,346us-gaap_AllowanceForDoubtfulAccountsReceivable $ 3,215us-gaap_AllowanceForDoubtfulAccountsReceivable $ 5,796us-gaap_AllowanceForDoubtfulAccountsReceivable
Charged to costs and expenses 3,994gncma_ProvisionForDoubtfulAccountsChargedtoCostsandExpenses 2,370gncma_ProvisionForDoubtfulAccountsChargedtoCostsandExpenses 3,649gncma_ProvisionForDoubtfulAccountsChargedtoCostsandExpenses
Charged to other accounts 0gncma_ProvisionForDoubtfulAccountsChargedtoOtherAccounts (446)gncma_ProvisionForDoubtfulAccountsChargedtoOtherAccounts (2,261)gncma_ProvisionForDoubtfulAccountsChargedtoOtherAccounts
Write-offs net of recoveries 1,798gncma_AllowanceforDoubtfulAccountsReceivableWriteoffsNetofRecoveries 2,793gncma_AllowanceforDoubtfulAccountsReceivableWriteoffsNetofRecoveries 3,969gncma_AllowanceforDoubtfulAccountsReceivableWriteoffsNetofRecoveries
Balance at end of year $ 4,542us-gaap_AllowanceForDoubtfulAccountsReceivable $ 2,346us-gaap_AllowanceForDoubtfulAccountsReceivable $ 3,215us-gaap_AllowanceForDoubtfulAccountsReceivable