| **Tower Sale and Leaseback (Balance Sheet Impacts) (Details) - USD ($) $ in Thousands** | Dec. 31, 2016 | Dec. 31, 2015 |
|-----------------------------------------------------------------------------------------|---------------|---------------|
| **Tower Sale and Leaseback [Abstract]**                                                 |          |          |
| Property and equipment                                                                  | $ 18,792      |          |
| Tower obligation                                                                        | $ 87,653      | $ 0           |

| X                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tower Sale and Leaseback  [Abstract]
+ References
No definition available.
+ Details**Name:**gncma_TowerSaleandLeasebackAbstract**Namespace Prefix:**gncma_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The liability due as of the balance sheet date for leasebacks accounted for using the deposit method.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 40 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6456341&loc=d3e50796-112755Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6452660&loc=d3e36991-112694
+ Details**Name:**us-gaap_SaleLeasebackTransactionAmountDueUnderFinancingArrangement**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The net book value of the asset(s) sold in connection with the sale of the property to another party and lease back to the seller.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 40 -Section 55 -Paragraph 52 -URI http://asc.fasb.org/extlink&oid=28365400&loc=d3e51843-112757Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 40 -Section 55 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=28365400&loc=d3e51831-112757Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 40 -Section 55 -Paragraph 51 -URI http://asc.fasb.org/extlink&oid=28365400&loc=d3e51840-112757
+ Details**Name:**us-gaap_SaleLeasebackTransactionNetBookValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |