|**Asset Retirement Obligations (Tables)**                     | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
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|**Asset Retirement Obligation Disclosure [Abstract]**         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Schedule of Change in Asset Retirement Obligation             | The following table presents the changes in carrying amounts of the asset retirement obligations associated with our working interests in crude oil and natural gas properties:Amount(in thousands)Balance at December 31, 2017$87,306Obligations incurred with development activities2,147Obligations incurred with acquisition4,326Accretion expense3,773Revisions in estimated cash flows754Obligations discharged with asset retirements and divestiture(9,593)Balance at September 30, 201888,713Current portion(16,006)Long-term portion$72,707
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|                                                              | Amount                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|                                                              | (in thousands)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Balance at December 31, 2017                                  | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Obligations incurred with development activities              | 2,147                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Obligations incurred with acquisition                         | 4,326                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Accretion expense                                             | 3,773                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Revisions in estimated cash flows                             | 754                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Obligations discharged with asset retirements and divestiture | (9,593                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Balance at September 30, 2018                                 | 88,713                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Current portion                                               | (16,006                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Long-term portion                                             | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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