SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ADDITIONAL INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2019	Oct. 11, 2019	Oct. 01, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Loans receivable in other current assets	$ 12,600			$ 12,400
Loans receivable in other noncurrent assets	39,600			$ 38,800
Operating lease assets	87,793
Operating lease, liability	$ 86,714
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
Other noncurrent assets
Finite-Lived Intangible Assets [Line Items]
Capitalized software costs	$ 9,100
Operation and maintenance
Finite-Lived Intangible Assets [Line Items]
Capitalized software costs	$ 1,500