| **Financial Instruments (Assets measured at fair value on a recurring basics) (Details) (Fair Value, Inputs, Level 1, USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                                   | Dec. 31, 2013                                                                                                                                   |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------|
| Fair Value, Inputs, Level 1                                                                                                                                            |                                                                                                                                            |                                                                                                                                            |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                       |                                                                                                                                            |                                                                                                                                            |
| Deferred compensation plan assets (mutual funds)                                                                                                                       | $ 2,068gncma_DeferredCompensationAssetsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member | $ 2,183gncma_DeferredCompensationAssetsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member |
| Total assets at fair value                                                                                                                                             | $ 2,068us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                   | $ 2,183us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the fair value of deferred compensation liabilities. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.
+ References
No definition available.
+ Details**Name:**gncma_DeferredCompensationAssetsFairValueDisclosure**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
+ Details**Name:**us-gaap_AssetsFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |