INCOME TAXES - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Tax Credit Carryforward [Line Items]
Deferred tax assets	$ 12,691
Deferred tax liabilities	(13,328)
Net deferred income tax liability	(637)
Net income (loss)	(2,376)	$ 131,608	$ 76,417
MGE Niagara Resorts
Tax Credit Carryforward [Line Items]
Deferred tax assets	12,163
Deferred tax liabilities	(13,300)
Net income (loss)	(45,900)
Other
Tax Credit Carryforward [Line Items]
Deferred tax assets	528
Deferred tax liabilities	$ (28)