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Summary Of Significant Accounting Policies (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2020
Mar. 31, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Sep. 30, 2019
Sep. 30, 2020
Mar. 31, 2019
Sep. 30, 2018
Summary Of Significant Accounting Policies [Line Items]                  
Gas Stored Underground $ 14,356     $ 14,356   $ 36,632      
Capitalized Costs Oil and Gas Producing Activities Net $ 1,600,000     1,600,000   1,700,000      
Capitalized costs of unproved properties excluded from amortization       $ 81,200   53,500      
Full cost ceiling test discount factor 10.00%     10.00%          
Impairment of Oil and Gas Producing Properties $ 18,236   $ 0 $ 195,997 $ 0        
Increase estimated future net cash flows $ 125,600 $ 32,700              
Antidilutive securities 513,428   120,546 513,180 122,327        
Retained Earnings (Accumulated Deficit) $ 1,177,650 1,176,870 $ 1,262,867 $ 1,177,650 $ 1,262,867 1,272,601   $ 1,236,657 $ 1,098,900
Earnings Reinvested in The Business [Member]                  
Summary Of Significant Accounting Policies [Line Items]                  
Retained Earnings (Accumulated Deficit) 1,177,650 1,176,870 $ 1,262,867 1,177,650 $ 1,262,867 1,272,601   1,236,657 1,098,900
Reserve For Gas Replacement [Member]                  
Summary Of Significant Accounting Policies [Line Items]                  
Gas Stored Underground 10,622     10,622   0      
Impairment of Oil and Gas Producing Properties [Member]                  
Summary Of Significant Accounting Policies [Line Items]                  
Deferred Income Tax Benefit $ 5,000     $ 53,500          
Non-performance Based Restricted Stock Units [Member]                  
Summary Of Significant Accounting Policies [Line Items]                  
Share based compensation other than options grants in period       150,839          
Granted in fiscal year, weighted average grant date fair value       $ 40.38          
Performance Shares [Member]                  
Summary Of Significant Accounting Policies [Line Items]                  
Share based compensation other than options grants in period       254,608          
Granted in fiscal year, weighted average grant date fair value       $ 43.32          
Subsequent Event [Member] | Reserve For Gas Replacement [Member]                  
Summary Of Significant Accounting Policies [Line Items]                  
Gas Stored Underground             $ 0    
Guidance for Hedging [Member] | Cumulative Effect of Adoption of Authoritative Guidance [Member]                  
Summary Of Significant Accounting Policies [Line Items]                  
Retained Earnings (Accumulated Deficit)   0       (950)   0 0
Increase (Decrease) to Accumulated Other Comprehensive Income           1,000      
Guidance for Hedging [Member] | Cumulative Effect of Adoption of Authoritative Guidance [Member] | Earnings Reinvested in The Business [Member]                  
Summary Of Significant Accounting Policies [Line Items]                  
Retained Earnings (Accumulated Deficit)           (950)      
Guidance for Recognition and Measurement of Financial Assets and Liabilities [Member] | Cumulative Effect of Adoption of Authoritative Guidance [Member]                  
Summary Of Significant Accounting Policies [Line Items]                  
Retained Earnings (Accumulated Deficit)   0       0   0 7,437
Increase (Decrease) to Accumulated Other Comprehensive Income                 (7,400)
Guidance for Recognition and Measurement of Financial Assets and Liabilities [Member] | Cumulative Effect of Adoption of Authoritative Guidance [Member] | Earnings Reinvested in The Business [Member]                  
Summary Of Significant Accounting Policies [Line Items]                  
Retained Earnings (Accumulated Deficit)                 7,437
Guidance for Reclassification of Stranded Tax Effects [Member] | Cumulative Effect of Adoption of Authoritative Guidance [Member]                  
Summary Of Significant Accounting Policies [Line Items]                  
Retained Earnings (Accumulated Deficit)   $ 0       $ 0   $ 0 10,406
Increase (Decrease) to Accumulated Other Comprehensive Income                 (10,400)
Guidance for Reclassification of Stranded Tax Effects [Member] | Cumulative Effect of Adoption of Authoritative Guidance [Member] | Earnings Reinvested in The Business [Member]                  
Summary Of Significant Accounting Policies [Line Items]                  
Retained Earnings (Accumulated Deficit)                 $ 10,406