| **Receivables and Allowance for Doubtful Receivables (Receivables by Type) (Details) - USD ($) $ in Thousands** | Dec. 31, 2016 | Dec. 31, 2015 |
|-----------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |          |          |
| Receivables                                                                                                     | $ 219,794     | $ 208,384     |
| Trade Accounts Receivable                                                                                       |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |          |          |
| Receivables                                                                                                     | 218,491       | 205,645       |
| Other Receivable                                                                                                |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |          |          |
| Receivables                                                                                                     | $ 1,303       | $ 2,739       |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amounts due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer) for goods or services (including trade receivables) that have been delivered or sold in the normal course of business.
+ References
Reference 1: 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(3)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(5)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910
+ Details**Name:**us-gaap_AccountsReceivableGrossCurrent**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=gncma_OtherReceivableMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |