| **Receivables and Allowance for Doubtful Receivables (Narrative) (Details) (USD $)In Millions, unless otherwise specified** |         |        |        |        | 12 Months Ended | 1 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------|---------|--------|--------|--------|-----------------|----------------|
||
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                            |    |   |   |   |            |           |
| Revenue support from regulatory agency, percentage                                                                          |    |   | 19.00% | 19.00% |            |           |
| Receivables net current                                                                                                     | $ 124.3 |   | $ 70.1 |   |            |           |
| Contribution receivables                                                                                                    |    | 14.2   |   |   |            |           |
| Proceeds from regulatory agency                                                                                             |    |   |   |   | $ 1.6           | $ 18.5         |

| X                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Proceeds from Regulatory Agency
+ References
No definition available.
+ Details**Name:**gncma_ProceedsfromRegulatoryAgency**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Public Utilities, Revenue Support From Regulatory Agency, Percentage
+ References
No definition available.
+ Details**Name:**gncma_PublicUtilitiesRevenueSupportFromRegulatoryAgencyPercentage**Namespace Prefix:**gncma_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Receivables Net, Contribution
+ References
No definition available.
+ Details**Name:**gncma_ReceivablesNetContribution**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AccountsNotesAndLoansReceivableLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The total amount due to the entity within one year of the balance sheet date (or one operating cycle, if longer) from outside sources, including trade accounts receivable, notes and loans receivable, as well as any other types of receivables, net of allowances established for the purpose of reducing such receivables to an amount that approximates their net realizable value.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 3 -Subparagraph a -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 210.5-02.3,4) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 4 -Article 5
+ Details**Name:**us-gaap_ReceivablesNetCurrent**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |