v3.26.1
Summary of Significant Accounting Policies (Narrative) (Details)
$ / shares in Units, $ in Thousands
 3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2026
USD ($)
shares
Sep. 30, 2025
USD ($)
segment
Mar. 31, 2025
shares
Dec. 31, 2024
USD ($)
Mar. 31, 2026
USD ($)
segment
$ / shares
shares
Mar. 31, 2025
shares
Sep. 30, 2025
USD ($)
Sep. 30, 2026
USD ($)
Summary Of Significant Accounting Policies [Line Items]                
Number of reportable segments | segment         3      
Gas Stored Underground $ 4,768 $ 33,468     $ 4,768   $ 33,468  
Capitalized costs relating to exploration and production activities, net $ 2,590,000 $ 2,460,000     2,590,000   2,460,000  
Capitalized costs of unproved properties excluded from amortization         $ 109,300   $ 112,400  
Full cost ceiling test discount factor (as a percent) 10.00%       10.00%      
Amount full cost ceiling exceeds book value of exploration and production properties $ 1,600,000       $ 1,600,000      
Impairment of exploration and production properties       $ 108,300        
Increase estimated future net cash flows         $ 59,500      
Antidilutive securities (in shares) | shares 99   30   2,072 0    
Impairment of Exploration and Production Properties                
Summary Of Significant Accounting Policies [Line Items]                
Deferred income tax benefit       $ 29,200        
Exploration and Production & Gathering Segments                
Summary Of Significant Accounting Policies [Line Items]                
Number of reportable segments | segment   1            
Reserve For Gas Replacement                
Summary Of Significant Accounting Policies [Line Items]                
Gas Stored Underground $ 47,400       $ 47,400      
Reserve For Gas Replacement | Forecast                
Summary Of Significant Accounting Policies [Line Items]                
Gas Stored Underground               $ 0
Vesting of Performance Shares                
Summary Of Significant Accounting Policies [Line Items]                
Share based compensation other than options grants in period (in shares) | shares         137,995      
Granted in fiscal year, weighted average grant date fair value (in USD per share) | $ / shares         $ 62.07      
Relative shareholder return, performance cycle (in years)         3 years      
Restricted Stock Units                
Summary Of Significant Accounting Policies [Line Items]                
Share based compensation other than options grants in period (in shares) | shares         128,755      
Granted in fiscal year, weighted average grant date fair value (in USD per share) | $ / shares         $ 77.75