Details of Selected Consolidated Balance Sheet Captions - Other Non-Operating Income (Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Equity component of AFUDC	$ 23,493	$ 11,165	$ 0
Performance-based rate program	6,771	6,737	6,745
Pension and other postretirement non-service credit (cost)	(3,189)	3,016	(5,770)
Interest income	2,932	4,160	1,450
Community support spending	(11,728)	(4,771)	(6,053)
Miscellaneous	(11,108)	(12,903)	(6,516)
Other non-operating income (expense)	$ 7,171	$ 7,404	$ (10,144)