| **Related Party Transactions (Narrative) (Details) - USD ($)**                                        |           | 1 Months Ended | 5 Months Ended | 12 Months Ended |             |             |
|-------------------------------------------------------------------------------------------------------|-----------|----------------|----------------|-----------------|-------------|-------------|
||
| Immediate Family Member of Management or Principal Owner [Member] | Property                          |      |           |           |            |        |        |   |
| **Related Party Transaction [Line Items]**                                                            |      |           |           |            |        |        |   |
| Capital lease obligations                                                                             |      |           |           |            |        |        | $ 900,000 |
| Immediate Family Member of Management or Principal Owner [Member] | Capital Lease Obligation Addition |      |           |           |            |        |        |   |
| **Related Party Transaction [Line Items]**                                                            |      |           |           |            |        |        |   |
| Capital lease obligations                                                                             |      |           |           |            |        | $ 1,300,000 |   |
| Chief Executive Officer [Member] | Second Aircraft                                                    |      |           |           |            |        |        |   |
| **Related Party Transaction [Line Items]**                                                            |      |           |           |            |        |        |   |
| Termination period on lease                                                                           | 12 months |           |           |            |        |        |   |
| Monthly lease payment                                                                                 | $ 132,000 |           |           |            |        |        |   |
| Capital lease deposit                                                                                 |      |           |           |            | $ 1,500,000 |        |   |
| Deposit termination period                                                                            |      |           |           |            | 6 months    |        |   |
| ACS                                                                                                   |      |           |           |            |        |        |   |
| **Related Party Transaction [Line Items]**                                                            |      |           |           |            |        |        |   |
| Payments to related party                                                                             |      | $ 6,200,000    | $ 25,100,000   | $ 62,900,000    |        |        |   |
| Receipts from Related Parties                                                                         |      | $ 8,100,000    | $ 23,900,000   | $ 50,900,000    |        |        |   |

| X                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Capital Lease, Monthly Lease Payment
+ References
No definition available.
+ Details**Name:**gncma_CapitalLeaseMonthlyLeasePayment**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Capital Lease, Termination Period
+ References
No definition available.
+ Details**Name:**gncma_CapitalLeaseTerminationPeriod**Namespace Prefix:**gncma_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cash receipts from related parties for the period.
+ References
No definition available.
+ Details**Name:**gncma_ReceiptsfromRelatedParties**Namespace Prefix:**gncma_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Security Deposit, Termination Period
+ References
No definition available.
+ Details**Name:**gncma_SecurityDepositTerminationPeriod**Namespace Prefix:**gncma_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7
+ Details**Name:**us-gaap_CapitalLeaseObligations**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| 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.
+ Details**Name:**us-gaap_RelatedPartyTransactionLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Purchases during the period (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.
+ References
No definition available.
+ Details**Name:**us-gaap_RelatedPartyTransactionPurchasesFromRelatedParty**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of an asset, typically cash, provided to a counterparty to provide certain assurance of performance by the entity pursuant to the terms of a written or oral agreement, such as a lease.
+ 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 210.5-02.8,17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_SecurityDeposit**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_ImmediateFamilyMemberOfManagementOrPrincipalOwnerMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_LandBuildingsAndImprovementsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_CapitalAdditionsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_ChiefExecutiveOfficerMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_PropertyPlantAndEquipmentByTypeAxis=gncma_AirTransportationEquipmentTwoMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=gncma_AlaskaCommunicationsSystemsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |