Details of Selected Financial Statement Captions - Other Non-Operating Income (Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Balance Sheet Related Disclosures [Abstract]
Equity component of AFUDC	$ 64,019	$ 45,505	$ 32,749
Performance-based rate program	7,093	8,327	6,362
Pension and other postretirement non-service credit (cost)	8,955	8,337	(19,238)
Interest income	7,207	2,781	2,144
Community support spending	(12,027)	(16,357)	(14,460)
Unrealized gains (losses) on equity securities	1,406	(7,737)	(860)
Miscellaneous	(6,878)	(7,119)	(8,842)
Other non-operating income (expense)	$ 69,775	$ 33,737	$ (2,145)