| **Allowance for Doubtful Receivables (Allowance for Doubtful Receivables Rollforward) (Details) (USD $)In Thousands, unless otherwise specified** | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Allowance for Doubtful Accounts Receivable [Roll Forward]**                                                                                     |            |   |   |
| Balance at beginning of year                                                                                                                      | $ 3,215         | $ 5,796 | $ 9,189 |
| Charged to costs and expenses                                                                                                                     | 2,370           | 3,649 | 4,294 |
| Charged to other accounts                                                                                                                         | (446)           | (2,261) | (29) |
| Write-offs net of recoveries                                                                                                                      | 2,793           | 3,969 | 7,658 |
| Balance at end of year                                                                                                                            | $ 2,346         | $ 3,215 | $ 5,796 |

| X                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of direct write-downs of receivables charged against the allowance for doubtful accounts, net of recoveries.
+ References
No definition available.
+ Details**Name:**gncma_AllowanceforDoubtfulAccountsReceivableWriteoffsNetofRecoveries**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Provision For Doubtful Accounts, Charged to Costs and Expenses
+ References
No definition available.
+ Details**Name:**gncma_ProvisionForDoubtfulAccountsChargedtoCostsandExpenses**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of allowance for doubtful accounts increase which was not charged to operations. Specifics of where it was charged is discussed in narrative to the footnote.
+ References
No definition available.
+ Details**Name:**gncma_ProvisionForDoubtfulAccountsChargedtoOtherAccounts**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
For an unclassified balance sheet, a valuation allowance for receivables due a company that are expected to be uncollectible.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 4 -Article 5Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=28368275&loc=d3e5074-111524Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.4) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_AllowanceForDoubtfulAccountsReceivable**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |