Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Summary of Significant Accounting Policies [Line Items]
Weather impact on revenues tempered by WNA, period	8 months
Above/below average weather normalized usage, surcharge calculation period	12 months
Capitalized costs relating to exploration and production activities, net	$ 2,276,590	$ 2,446,226
Full cost ceiling test discount factor (as a percent)	10.00%
Impairment Of Exploration And Production Properties	$ 463,700
Increase (decrease) estimated future net cash flows	428,500	38,800	$ (1,000,000)
Depreciation, depletion and amortization for exploration and production properties	270,648	235,694	$ 202,418
Goodwill	5,476	5,476
Prior service cost	400	400
Accumulated losses	70,900	59,300
Gas stored underground	$ 35,055	32,509
Rate recovery period	4 years
Balanced billing, estimated annual usage, payment period	12 months
Customer advances	$ 19,373	21,003
Customer security deposits	$ 36,265	$ 28,764
Antidilutive securities (in shares)	569	3,888	2,858
Impairment Of Exploration And Production Properties
Summary of Significant Accounting Policies [Line Items]
Deferred income tax benefit	$ 127,300
Amount Exceeds LIFO Basis
Summary of Significant Accounting Policies [Line Items]
Gas stored underground	8,300
LIFO Method
Summary of Significant Accounting Policies [Line Items]
Gas stored underground	$ 35,000