INCOME TAXES - INCOME TAX RECONCILIATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory income tax expense	$ 27,668	$ 44,014	$ 52,643
Change resulting from:
Investment/production tax credits	(66,035)	(21,446)	(34,526)
Cost of removal of assets placed in service prior to 1981	(6,349)	(5,829)	(6,886)
AFUDC equity	(2,313)	(2,117)	(2,624)
State income taxes, net of federal benefit	7,707	7,092	8,222
Basis adjustment of solar assets due to ITC	6,500	1,080	4,256
Tax Act - utility excess deferred income taxes amortized	(3,573)	(1,786)	0
Tax Act - nonutility excess deferred income taxes	0	(59,627)	0
Tax Act - utility excess deferred income taxes refunded to customers	0	(14,323)	0
Other	(1,356)	(843)	(2,742)
Income tax (benefit) provision	$ (37,751)	$ (53,785)	$ 18,343
Effective income tax rate	(28.70%)	(29.90%)	12.20%
Federal statutory rate	21.00%	24.50%	35.00%