SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ADDITIONAL INFORMATION (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2020 USD ($) plan	Oct. 11, 2019 USD ($)	Oct. 01, 2019 USD ($)	Sep. 30, 2019 USD ($)
Finite-Lived Intangible Assets [Line Items]
Loans receivable in other current assets	$ 13,100	$ 13,100			$ 12,400
Loans receivable in other noncurrent assets	38,000	$ 38,000			38,800
Number of plans | plan		2
Number of medical and life insurance plans | plan		2
Operating lease assets	102,395	$ 102,395
Operating lease, liability	100,869	$ 100,869
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
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)	1,100	$ 2,600
Other Noncurrent Assets
Finite-Lived Intangible Assets [Line Items]
Capitalized software costs	3,200	3,200			1,700
Construction in Progress
Finite-Lived Intangible Assets [Line Items]
Capitalized software costs	$ 9,100	$ 9,100			$ 4,800