SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES (Details) - Reserves for uncollectible accounts - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balances at Beginning of Year	$ 11,715	$ 12,265	$ 20,785
Charges to Costs and Expenses	4,592	976	4,396
Deductions from Reserves	(6)	1,526	12,916
Balances at End of Year	$ 16,313	$ 11,715	12,265
Cowlitz Project
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Write-off of receivables			$ 10,300