| **Fair Value Measurements and Derivative Instruments (Carrying amounts and fair value of the financial instruments) (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                            | Dec. 31, 2014                                                                                                                                |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------|
| Senior Notes Due 2021 | Line of Credit                                                                                                                                          |                                                                                                                                                                                     |                                                                                                                                         |
| **Debt Instrument [Line Items]**                                                                                                                                                |                                                                                                                                                                                     |                                                                                                                                         |
| Debt stated percentage                                                                                                                                                          | 6.75%us-gaap_DebtInstrumentInterestRateStatedPercentage/ us-gaap_DebtInstrumentAxis= gncma_SeniorNotesDueTwentyTwentyOneMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_LineOfCreditMember |                                                                                                                                         |
| Senior Notes Due 2019 | Line of Credit                                                                                                                                          |                                                                                                                                                                                     |                                                                                                                                         |
| **Debt Instrument [Line Items]**                                                                                                                                                |                                                                                                                                                                                     |                                                                                                                                         |
| Debt stated percentage                                                                                                                                                          | 8.63%us-gaap_DebtInstrumentInterestRateStatedPercentage/ us-gaap_DebtInstrumentAxis= gncma_SeniorNotesDueTwentyNineteenMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_LineOfCreditMember  |                                                                                                                                         |
| Carrying Amount                                                                                                                                                                 |                                                                                                                                                                                     |                                                                                                                                         |
| **Debt Instrument [Line Items]**                                                                                                                                                |                                                                                                                                                                                     |                                                                                                                                         |
| Current and long-term debt                                                                                                                                                      | $ 1,326,172us-gaap_DebtInstrumentFairValue/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CarryingReportedAmountFairValueDisclosureMember                                             | $ 1,036,678us-gaap_DebtInstrumentFairValue/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CarryingReportedAmountFairValueDisclosureMember |
| Fair Value                                                                                                                                                                      |                                                                                                                                                                                     |                                                                                                                                         |
| **Debt Instrument [Line Items]**                                                                                                                                                |                                                                                                                                                                                     |                                                                                                                                         |
| Current and long-term debt                                                                                                                                                      | $ 1,376,834us-gaap_DebtInstrumentFairValue/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                                                | $ 1,055,952us-gaap_DebtInstrumentFairValue/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember    |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13476-108611
+ Details**Name:**us-gaap_DebtInstrumentFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Contractual interest rate for funds borrowed, under the debt agreement.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_DebtInstrumentInterestRateStatedPercentage**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DebtInstrumentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DebtInstrumentAxis=gncma_SeniorNotesDueTwentyNineteenMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DebtInstrumentAxis=gncma_SeniorNotesDueTwentyTwentyOneMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |