|**Business Segments (Tables)**             | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|------------------------------------------ | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Segment Reporting [Abstract]**           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Schedule of Segment Reporting Information  | The following tables present our segment information:Three Months Ended March 31,20142013(in thousands)Segment revenues:Oil and gas exploration and production$103,325$58,156Gas marketing26,93713,670Total revenues$130,262$71,826Segment income (loss) before income taxes:Oil and gas exploration and production$34,006$(33,702)Gas marketing67(66)Unallocated(37,587)(29,679)Loss before income taxes$(3,514)$(63,447)March 31, 2014December 31, 2013(in thousands)Segment assets:Oil and gas exploration and production$1,965,354$1,937,251Gas marketing28,36620,342Unallocated71,44867,610Total assets$2,065,168$2,025,203
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|                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|                                           | Three Months Ended March 31,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|                                           | 2014                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|                                           | (in thousands)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Segment revenues:                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Oil and gas exploration and production     | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Gas marketing                              | 26,937                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Total revenues                             | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Segment income (loss) before income taxes: |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Oil and gas exploration and production     | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Gas marketing                              | 67                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Unallocated                                | (37,587                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Loss before income taxes                   | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
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|                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|                                           | March 31, 2014                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|                                           | (in thousands)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Segment assets:                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Oil and gas exploration and production     | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Gas marketing                              | 28,366                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Unallocated                                | 71,448                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Total assets                               | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
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