Details of Selected Consolidated Balance Sheet Captions Other Non-Operating Income (Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Other Income and Expenses [Abstract]
Equity Component of AFUDC	$ 11,165	$ 0	$ 0
Performance-based rate program	6,737	6,745	9,240
Pension and Other Postretirement Non-Service Cost	3,016	(5,770)	(8,469)
Interest Income	4,160	1,450	1,390
Charitable Contributions	(4,771)	(6,053)	(4,413)
Gain (loss) on assets held in trusts	(1,349)	0	0
Miscellaneous Non Operating Income Expense	(11,554)	(6,516)	(9,100)
Miscellaneous expense (benefit)	$ 7,404	$ (10,144)	$ (11,352)