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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

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