| **Fair Value Measurements and Derivative Instruments (Assets and liabilities measured at fair value on a recurring basis) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands** | Sep. 30, 2016 | Dec. 31, 2015 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                              |          |          |
| Deferred compensation plan assets (mutual funds)                                                                                                                                                              | $ 1,462       | $ 1,728       |
| Derivative stock appreciation rights                                                                                                                                                                          | 16,980        | 32,820        |
| Fair Value, Inputs, Level 1 [Member]                                                                                                                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                              |          |          |
| Deferred compensation plan assets (mutual funds)                                                                                                                                                              | 1,462         | 1,728         |
| Derivative stock appreciation rights                                                                                                                                                                          | 0             | 0             |
| Fair Value, Inputs, Level 2 [Member]                                                                                                                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                              |          |          |
| Deferred compensation plan assets (mutual funds)                                                                                                                                                              | 0             | 0             |
| Derivative stock appreciation rights                                                                                                                                                                          | 0             | 0             |
| Fair Value, Inputs, Level 3 [Member]                                                                                                                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                              |          |          |
| Deferred compensation plan assets (mutual funds)                                                                                                                                                              | 0             | 0             |
| Derivative stock appreciation rights                                                                                                                                                                          | $ 16,980      | $ 32,820      |

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| - 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 |
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| - Definition
Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13495-108611Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312Reference 4: 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=73719730&loc=d3e13433-108611Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958
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| 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                                                                                                                                                                                |
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| X                                                                                                                                                                          |
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