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Summary of Significant Accounting Policies - Narrative (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2025 USD ($) segment | Dec. 31, 2024 USD ($) | Sep. 30, 2025 USD ($) shares | Sep. 30, 2024 USD ($) shares | Sep. 30, 2023 USD ($) shares
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Weather impact on revenues tempered by WNA, period |  |  | 8 months |  |
WNA pilot program, period |  |  | 5 years |  |
Above/below average weather normalized usage, surcharge calculation period |  |  | 12 months |  |
Capitalized costs relating to exploration and production activities, net | $ 2,457,411 |  | $ 2,457,411 | $ 2,276,590 |
Full cost ceiling test discount factor (as a percent) | 10.00% |  | 10.00% |  |
Amount full cost ceiling exceeds book value of oil and gas properties | $ 1,100,000 |  | $ 1,100,000 |  |
Impairment of exploration and production properties |  | $ 108,300 |  | 463,700 |
Increase (decrease) estimated future net cash flows |  |  | 261,000 | 428,500 | $ 38,800
Depreciation, depletion and amortization for exploration and production properties |  |  | 261,712 | 270,648 | $ 235,694
Goodwill | 5,476 |  | 5,476 | 5,476 |
Prior service cost | 400 |  | 400 | 400 |
Accumulated losses | 78,800 |  | 78,800 | 70,900 |
Gas stored underground | $ 33,468 |  | $ 33,468 | 35,055 |
Rate recovery period | 3 years |  | 3 years |  |
Balanced billing, estimated annual usage, payment period |  |  | 12 months |  |
Customer advances | $ 17,188 |  | $ 17,188 | 19,373 |
Customer security deposits | $ 29,853 |  | $ 29,853 | $ 36,265 |
Antidilutive securities (in shares) | shares |  |  | 3,228 | 569 | 3,888
Exploration and Production & Gathering Segments |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Number of operating segments | segment | 1 |  |  |  |
LIFO Method |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Gas stored underground | $ 33,500 |  | $ 33,500 |  |
Amount Exceeds LIFO Basis |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Gas stored underground | $ 18,700 |  | $ 18,700 |  |
Impairment Of Exploration And Production Properties |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Deferred income tax benefit |  | $ 29,200 |  | $ 127,300 |

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