SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ADDITIONAL INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Oct. 11, 2019	Oct. 01, 2019	Sep. 30, 2019
Finite-Lived Intangible Assets [Line Items]
Loans receivable in other current assets	$ 13,000		$ 13,000				$ 12,400
Loans receivable in other noncurrent assets	36,300		36,300				38,800
Operating lease assets	101,657		101,657				0
Operating lease, liability	100,576		$ 100,576
Leaf River Energy Center LLC
Finite-Lived Intangible Assets [Line Items]
Operating lease assets					$ 21,600
Operating lease, liability					$ 21,600
Accounting Standards Update 2016-02
Finite-Lived Intangible Assets [Line Items]
Operating lease assets						$ 67,100
Operating lease, liability						$ 67,100
Minimum
Finite-Lived Intangible Assets [Line Items]
Loans receivable, term			2 years
Maximum
Finite-Lived Intangible Assets [Line Items]
Loans receivable, term			10 years
Operation and maintenance
Finite-Lived Intangible Assets [Line Items]
Capitalized Computer Software, Period Increase (Decrease)	1,900	$ 3,700	$ 4,500	$ 3,700
Other Noncurrent Assets
Finite-Lived Intangible Assets [Line Items]
Capitalized software costs	4,300		4,300				1,700
Construction in Progress
Finite-Lived Intangible Assets [Line Items]
Capitalized software costs	$ 12,200		$ 12,200				$ 4,800