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Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended
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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 |  |

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