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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, 2018
Dec. 31, 2017
Sep. 30, 2018
Sep. 30, 2019
Summary Of Significant Accounting Policies [Line Items]        
Pension and Postretirement Benefit Costs in Other Income and Deductions $ 7,400 $ 7,500    
Gas stored underground (30,336)   $ (37,813)  
Capitalized costs of unproved properties excluded from amortization $ 58,500   62,200  
Full cost ceiling test discount factor 10.00%      
Amount Full Cost Ceiling Exceeds Book Value Of Oil And Gas Properties $ 776,300      
Decrease estimated future net cash flows $ 44,400      
Antidilutive securities 318,106 157,603    
Reserve For Gas Replacement [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Gas stored underground $ 2,025   $ 0  
Non-performance Based Restricted Stock Units [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Share based compensation other than options grants in period 111,108      
Granted in fiscal year, weighted average grant date fair value $ 49.72      
Performance Shares [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Share based compensation other than options grants in period 244,734      
Granted in fiscal year, weighted average grant date fair value $ 55.67      
Subsequent Event [Member] | Reserve For Gas Replacement [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Gas stored underground       $ 0
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 for Financial Assets and Liabilities $ 7,437 $ 0