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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, 2017
Dec. 31, 2016
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2018
Jun. 13, 2016
Summary Of Significant Accounting Policies [Line Items]            
Cumulative Effect of Adoption of Authoritative Guidance for Stock-Based Compensation $ 31,900          
Gas stored underground (24,139)   $ (35,689)      
Capitalized costs of unproved properties excluded from amortization $ 77,100   80,900      
Full cost ceiling test discount factor 10.00%          
Amount Full Cost Ceiling Exceeds Book Value Of Oil And Gas Properties $ 334,600          
Increase estimated future net cash flows 18,000          
Receivable from Sale of Oil and Gas Producing Properties 17,310 $ 20,795        
Net Proceeds from Sale of Oil and Gas Producing Properties $ 0 $ 5,759        
Antidilutive securities 157,603 317,686        
Seneca [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Cumulative Net Proceeds from Sale of Oil and Gas Producing Properties $ 163,900          
Net Proceeds from Sale of Oil and Gas Producing Properties     26,600 $ 137,300    
IOG-CRV Marcellus, LLC [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Partner Working Interest In Joint Wells           80.00%
Internal Rate of Return           15.00%
Reserve For Gas Replacement [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Gas stored underground $ 1,739   $ 0      
Non-performance Based Restricted Stock Units [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Share based compensation other than options grants in period 89,672          
Granted in fiscal year, weighted average grant date fair value $ 51.23          
Performance Shares [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Share based compensation other than options grants in period 208,588          
Granted in fiscal year, weighted average grant date fair value $ 50.95          
Subsequent Event [Member] | Reserve For Gas Replacement [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Gas stored underground         $ 0  
26 Percent Net Revenue Interest [Member] | Seneca [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Wells to be Developed           56
After Internal Rate of Return Achieved [Member] | Seneca [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Partner Working Interest In Joint Wells           85.00%
Extended Agreement [Member] | Seneca [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Partner Working and Net Revenue Interest           20.00%
Royalty Interest           7.50%
Partner Working Interest In Joint Wells           20.00%
Partner Net Revenue Interest in Joint Wells           26.00%
Extended Agreement [Member] | IOG-CRV Marcellus, LLC [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Wells to be Developed           75
Partner Amount Funded to Develop Joint Wells $ 267,100          
Partner Commitment to Develop Joint Wells           $ 305,000
20 Percent Net Revenue Interest [Member] | Seneca [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Wells to be Developed           19