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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
May 01, 2018
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2018
Mar. 31, 2017
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   $ 31,900          
Gas stored underground   $ (6,842)   (6,842)   $ (35,689)      
Capitalized costs of unproved properties excluded from amortization       $ 67,600   80,900      
Full cost ceiling test discount factor   10.00%   10.00%          
Amount Full Cost Ceiling Exceeds Book Value Of Oil And Gas Properties   $ 502,800   $ 502,800          
Increase estimated future net cash flows   $ 7,500              
Net Proceeds from Sale of Oil and Gas Producing Properties       $ 17,310 $ 26,554        
Antidilutive securities   685,338 157,554 316,159 158,211        
Seneca [Member]                  
Summary Of Significant Accounting Policies [Line Items]                  
Cumulative Net Proceeds from Sale of Oil and Gas Producing Properties   $ 181,200   $ 181,200          
Net Proceeds from Sale of Oil and Gas Producing Properties       17,300   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   34,332   $ 34,332   $ 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]                  
Summary Of Significant Accounting Policies [Line Items]                  
Net Proceeds from Sale of Oil and Gas Producing Properties $ 38,300                
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   $ 301,500   $ 301,500          
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