| **Subsequent Event (Details) (Senior Notes, USD $)** | 0 Months Ended                                                                                                                                                                                                                                            | 3 Months Ended                                                                                                                                                                                                                                      |
|------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| 6.875% Senior Notes due 2025 | Subsequent Event      |                                                                                                                                                                                                                                                      |                                                                                                                                                                                                                                                |
| **Subsequent Event [Line Items]**                    |                                                                                                                                                                                                                                                      |                                                                                                                                                                                                                                                |
| Debt face amount                                     | $ 450,000,000.0us-gaap_DebtInstrumentFaceAmount/ us-gaap_DebtInstrumentAxis= gncma_SeniorNotes6.875Percentdue2025Member/ us-gaap_LongtermDebtTypeAxis= us-gaap_SeniorNotesMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember          |                                                                                                                                                                                                                                                |
| Debt stated percentage                               | 6.875%us-gaap_DebtInstrumentInterestRateStatedPercentage/ us-gaap_DebtInstrumentAxis= gncma_SeniorNotes6.875Percentdue2025Member/ us-gaap_LongtermDebtTypeAxis= us-gaap_SeniorNotesMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember |                                                                                                                                                                                                                                                |
| Debt issuance price, percent                         | 99.105%gncma_DebtInstrumentIssuancePricePercent/ us-gaap_DebtInstrumentAxis= gncma_SeniorNotes6.875Percentdue2025Member/ us-gaap_LongtermDebtTypeAxis= us-gaap_SeniorNotesMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember          |                                                                                                                                                                                                                                                |
| Closing costs                                        | 7,700,000us-gaap_DebtIssuanceCosts/ us-gaap_DebtInstrumentAxis= gncma_SeniorNotes6.875Percentdue2025Member/ us-gaap_LongtermDebtTypeAxis= us-gaap_SeniorNotesMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember                       |                                                                                                                                                                                                                                                |
| Scenario, Forecast | Senior Notes Due 2019           |                                                                                                                                                                                                                                                      |                                                                                                                                                                                                                                                |
| **Subsequent Event [Line Items]**                    |                                                                                                                                                                                                                                                      |                                                                                                                                                                                                                                                |
| Loss on extinguishment of debt                       |                                                                                                                                                                                                                                                      | $ 27,700,000us-gaap_GainsLossesOnExtinguishmentOfDebt/ us-gaap_DebtInstrumentAxis= gncma_SeniorNotesDueTwentyNineteenMember/ us-gaap_LongtermDebtTypeAxis= us-gaap_SeniorNotesMember/ us-gaap_StatementScenarioAxis= us-gaap_ScenarioForecastMember |

| X                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Debt Instrument, Issuance Price, Percent
+ References
No definition available.
+ Details**Name:**gncma_DebtInstrumentIssuancePricePercent**Namespace Prefix:**gncma_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Face (par) amount of debt instrument at time of issuance.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400
+ Details**Name:**us-gaap_DebtInstrumentFaceAmount**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                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of debt issuance costs (for example, but not limited to, legal, accounting, broker, and regulatory fees).
+ References
No definition available.
+ Details**Name:**us-gaap_DebtIssuanceCosts**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 50 -Section 40 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6850294&loc=d3e12317-112629Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 50 -Section 40 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6850294&loc=d3e12355-112629
+ Details**Name:**us-gaap_GainsLossesOnExtinguishmentOfDebt**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_SubsequentEventLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |