Details of Selected Consolidated Balance Sheet Captions - Other Non-Operating Income (Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Balance Sheet Related Disclosures [Abstract]
Equity component of AFUDC	$ 45,505	$ 32,749	$ 23,493
Performance-based rate program	8,327	6,362	6,771
Pension and other postretirement non-service credit (cost)	8,337	(19,238)	(3,189)
Interest income	2,781	2,144	2,932
Community support spending	(16,357)	(14,460)	(11,728)
Unrealized losses on equity securities	(7,737)	(860)	(4,176)
Miscellaneous	(7,119)	(8,842)	(6,932)
Other non-operating income (expense)	$ 33,737	$ (2,145)	$ 7,171