| **Income Taxes (Statutory tax rate impact on Income tax expense) (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                         | 12 Months Ended                        |
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| **Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]**                                |                                   |                                   |   |   |   |   |   |
| Expected statutory tax expense                                                                                               |                                   |                                   |   |   | $ 24,246us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate | $ 14,939us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate | $ 7,437us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate |
| Impact of non-controlling interest attributable to non-tax paying entity                                                     |                                   |                                   |   |   | (18,255)us-gaap_IncomeTaxReconciliationMinorityInterestIncomeExpense | (7,977)us-gaap_IncomeTaxReconciliationMinorityInterestIncomeExpense | 0us-gaap_IncomeTaxReconciliationMinorityInterestIncomeExpense |
| State income taxes, net of federal expense                                                                                   |                                   |                                   |   |   | 948us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes | 1,690us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes | 1,770us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes |
| Income tax effect of nondeductible entertainment expenses                                                                    |                                   |                                   |   |   | 1,125us-gaap_IncomeTaxReconciliationNondeductibleExpenseMealsAndEntertainment | 1,045us-gaap_IncomeTaxReconciliationNondeductibleExpenseMealsAndEntertainment | 777us-gaap_IncomeTaxReconciliationNondeductibleExpenseMealsAndEntertainment |
| Income tax effect of nondeductible lobbying expenses                                                                         |                                   |                                   |   |   | 425gncma_IncomeTaxReconciliationNondeductibleExpenseLobbying | 369gncma_IncomeTaxReconciliationNondeductibleExpenseLobbying | 298gncma_IncomeTaxReconciliationNondeductibleExpenseLobbying |
| Income tax effect of nondeductible officer compensation                                                                      |                                   |                                   |   |   | 1,351us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost | 824us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost | 1,718us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost |
| Other, net                                                                                                                   |                                   |                                   |   |   | 189us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther | 67us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther | 88us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther |
| Income Tax Expense (Benefit), Total                                                                                          | $ 1,400us-gaap_IncomeTaxExpenseBenefit | $ 5,078us-gaap_IncomeTaxExpenseBenefit | $ 2,355us-gaap_IncomeTaxExpenseBenefit | $ 1,196us-gaap_IncomeTaxExpenseBenefit | $ 10,029us-gaap_IncomeTaxExpenseBenefit | $ 10,957us-gaap_IncomeTaxExpenseBenefit | $ 12,088us-gaap_IncomeTaxExpenseBenefit |

| X                                                                                                                                                                                                                                                                                                      |
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| - Definition
Income Tax Reconciliation, Nondeductible Expense, Lobbying
+ References
No definition available.
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| - Definition
Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.
+ References
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| - Definition
Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to noncontrolling interest income (loss) exempt from income taxes.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to nondeductible meals and entertainment expense.
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| - Definition
Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to other nondeductible expenses.
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| - Definition
Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to nondeductible equity-based compensation costs.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to state and local income tax expense (benefit).
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