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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