INCOME TAXES - INCOME TAX RECONCILIATION (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory income tax expense			$ 39,265	$ 27,668	$ 44,014
Change resulting from:
Investment/production tax credits			(42,220)	(66,035)	(21,446)
Cost of removal of assets placed in service prior to 1981			(5,362)	(6,349)	(5,829)
AFUDC equity			(4,933)	(2,313)	(2,117)
State income taxes, net of federal benefit			8,657	7,707	7,092
NJ Unitary method change			(15,345)	0	0
Basis adjustment of solar assets due to ITC			4,399	6,500	1,080
Valuation allowance			13,604	0	0
Tax Act - utility excess deferred income taxes amortized			(3,573)	(3,573)	(1,786)
Tax Act - nonutility excess deferred income taxes			0	0	(59,627)
Tax Act - utility excess deferred income taxes refunded to customers			0	0	(14,323)
Other			(1,436)	(1,356)	(843)
Income tax benefit			$ (6,944)	$ (37,751)	$ (53,785)
Effective income tax rate			(3.70%)	(28.70%)	(29.90%)
Federal statutory rate	21.00%	35.00%	21.00%	21.00%	24.50%
Effective income tax rate without the impact of the Tax Act					12.40%