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Summary Of Significant Accounting Policies (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2017
Mar. 31, 2016
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 $ 31,900   $ 31,900          
Gas stored underground $ (9,027)   (9,027)   $ (34,332)      
Capitalized costs of unproved properties excluded from amortization     $ 114,300   135,300      
Full cost ceiling test discount factor 10.00%   10.00%          
Amount Full Cost Ceiling Exceeds Book Value Of Oil And Gas Properties $ 201,000   $ 201,000          
Impairment of Oil and Gas Producing Properties 0 $ 397,443 0 $ 832,894        
Increase estimated future net cash flows $ 105,400              
Net Proceeds from Sale of Oil and Gas Producing Properties     $ 26,554 $ 104,938        
Antidilutive securities 157,554 386,626 158,211 451,291        
Seneca [Member]                
Summary Of Significant Accounting Policies [Line Items]                
Cumulative Net Proceeds from Sale of Oil and Gas Producing Properties $ 163,900   $ 163,900          
Net Proceeds from Sale of Oil and Gas Producing Properties     26,600   137,300      
Reduction in Property, Plant and Equipment 163,900   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 20,215   $ 20,215   $ 0      
Non-performance Based Restricted Stock Units [Member]                
Summary Of Significant Accounting Policies [Line Items]                
Share based compensation other than options grants in period     87,143          
Granted in fiscal year, weighted average grant date fair value     $ 52.13          
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    
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  
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 $ 251,400   $ 251,400          
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