Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Summary of Significant Accounting Policies [Line Items]
Weather impact on revenues tempered by WNC, period		8 months
Above/below average weather normalized usage, surcharge calculation period		12 months
Capitalized costs relating to oil and gas producing activities, net		$ 2,446,226	$ 1,947,535
Full cost ceiling test discount factor (as a percent)		10.00%
Amount full cost ceiling exceeds book value of oil and gas properties		$ 794,700
Increase (decrease) estimated future net cash flows		38,800	(1,000,000)	$ (76,100)
Depreciation, depletion and amortization for oil and gas properties		235,694	202,418	177,055
Goodwill		5,476	5,476
Prior service cost		400	400
Accumulated losses		59,300	53,200
Other post-retirement adjustment for regulatory proceeding, before tax	$ (7,400)	0	(7,351)	$ 0
Other post-retirement adjustment for regulatory proceeding, after tax	$ (5,800)		(5,807)
Gas stored underground		$ 32,509	32,364
Rate recovery period		4 years
Balanced billing, estimated annual usage, payment period		12 months
Customer advances		$ 21,003	26,108
Customer security deposits		$ 28,764	$ 24,283
Antidilutive securities (in shares)		3,888	2,858	320,222
Minimum | Stock Appreciation Rights (SARs)
Summary of Significant Accounting Policies [Line Items]
SARs exercisable, period		1 year
Maximum | Stock Appreciation Rights (SARs)
Summary of Significant Accounting Policies [Line Items]
SARs exercisable, period		10 years
Amount Exceeds LIFO Basis
Summary of Significant Accounting Policies [Line Items]
Gas stored underground		$ 3,700
LIFO Method
Summary of Significant Accounting Policies [Line Items]
Gas stored underground		$ 32,400