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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, 2016
Dec. 31, 2015
Sep. 30, 2016
Sep. 30, 2017
Jun. 13, 2016
Dec. 01, 2015
Summary Of Significant Accounting Policies [Line Items]            
Cumulative Effect of Adoption of Authoritative Guidance for Stock-Based Compensation $ 31,916 $ 0        
Gas stored underground (23,431)   $ (34,332)      
Capitalized costs of unproved properties excluded from amortization $ 126,700   135,300      
Full cost ceiling test discount factor 10.00%          
Amount Full Cost Ceiling Exceeds Book Value Of Oil And Gas Properties $ 71,500          
Impairment of Oil and Gas Producing Properties 0 435,451        
Increase estimated future net cash flows 169,300          
Net Proceeds from Sale of Oil and Gas Producing Properties $ 5,759 $ 10,574        
Antidilutive securities 317,686 394,205        
Receivable from Sale of Oil and Gas Producing Properties $ 20,795 $ 94,364        
Seneca [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Cumulative Net Proceeds from Sale of Oil and Gas Producing Properties 143,100          
Net Proceeds from Sale of Oil and Gas Producing Properties 5,800   137,300      
Receivable from Sale of Oil and Gas Producing Properties 20,800          
Reduction in Property, Plant and Equipment 163,900          
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 $ 1,700   $ 0      
Stock Options [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Stock options granted 0          
Restricted Shares [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Share based compensation other than options grants in period 0          
Stock Appreciation Right [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Share based compensation other than options grants in period 0          
Non-performance Based Restricted Stock Units [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Share based compensation other than options grants in period 85,643          
Granted in fiscal year, weighted average grant date fair value $ 52.12          
Performance Shares [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Share based compensation other than options grants in period 184,148          
Granted in fiscal year, weighted average grant date fair value $ 56.39          
Subsequent Event [Member] | Reserve For Gas Replacement [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Gas stored underground       $ 0    
Maximum [Member] | IOG-CRV Marcellus, LLC [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Wells to be Developed         82  
Initial Participation [Member] | IOG-CRV Marcellus, LLC [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Wells to be Developed           42
26 Percent Net Revenue Interest [Member] | Seneca [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Wells to be Developed         56  
Additional Option to Participate [Member] | IOG-CRV Marcellus, LLC [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Wells to be Developed         7  
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 Commitment to Develop Joint Wells         $ 325,000  
20 Percent Net Revenue Interest [Member] | Seneca [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Wells to be Developed         19