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Summary Of Significant Accounting Policies (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Sep. 30, 2019
Sep. 30, 2020
Summary Of Significant Accounting Policies [Line Items]        
Gas stored underground $ (29,991)   $ (36,632)  
Capitalized Costs Oil and Gas Producing Activities Net 1,700,000   1,700,000  
Capitalized costs of unproved properties excluded from amortization $ 57,900   53,500  
Full cost ceiling test discount factor 10.00%      
Amount Full Cost Ceiling Exceeds Book Value Of Oil And Gas Properties $ 59,100      
Increase estimated future net cash flows $ 9,100      
Antidilutive securities 733,617 318,106    
Reserve For Gas Replacement [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Gas stored underground $ 1,100   $ 0  
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]        
Summary Of Significant Accounting Policies [Line Items]        
Cumulative Effect of Adoption of Authoritative Guidance $ (1,000)      
Guidance for Recognition and Measurement of Financial Assets and Liabilities [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Cumulative Effect of Adoption of Authoritative Guidance   $ 7,400