Details of Selected Consolidated Balance Sheet Captions - Other Non-Operating Income (Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Equity component of AFUDC	$ 32,749	$ 23,493	$ 11,165
Performance-based rate program	6,362	6,771	6,737
Pension and other postretirement non-service credit (cost)	(19,238)	(3,189)	3,016
Interest income	2,144	2,932	4,160
Community support spending	(14,460)	(11,728)	(4,771)
Miscellaneous	(9,702)	(11,108)	(12,903)
Other non-operating income (expense)	$ (2,145)	$ 7,171	$ 7,404