|**Properties and Equipment (Tables)**                                                        | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
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|**Oil and Gas Property [Abstract]**                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Property, Plant and Equipment                                                                | The following table presents the components of properties and equipment, net of accumulated depreciation, depletion and amortization ("DD&A"):March 31, 2020December 31, 2019(in thousands)Properties and equipment, net:Crude oil and natural gas propertiesProved$7,131,659$6,241,780Unproved385,138403,379Total crude oil and natural gas properties7,516,7976,645,159Equipment and other64,49841,888Land and buildings26,66412,312Construction in progress615,073408,428Properties and equipment, at cost8,223,0327,107,787Accumulated DD&A(3,188,538)(3,012,585)Properties and equipment, net$5,034,494$4,095,202
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|                                                                                             | March 31, 2020                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|                                                                                             | (in thousands)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Properties and equipment, net:                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Crude oil and natural gas properties                                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Proved                                                                                       | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Unproved                                                                                     | 385,138                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Total crude oil and natural gas properties                                                   | 7,516,797                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Equipment and other                                                                          | 64,498                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Land and buildings                                                                           | 26,664                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Construction in progress                                                                     | 615,073                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Properties and equipment, at cost                                                            | 8,223,032                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Accumulated DD&A                                                                             | (3,188,538                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Properties and equipment, net                                                                | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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|Schedule of Aging of Capitalized Exploratory Well Costs [Table Text Block]                   | The following table presents the capitalized exploratory well cost pending determination of proved reserves and included in properties and equipment, net on the condensed consolidated balance sheets:Three Months Ended March 31,Year Ended December 31, 2019(in thousands, except for number of wells)Beginning balance$16,078$12,188Additions to capitalized exploratory well costs pending the determination of proved reserves11,40831,901Reclassifications to proved properties(20,311)(28,011)Ending balance$7,175$16,078Number of wells pending determination at period-end24                                
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|Beginning balance                                                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Additions to capitalized exploratory well costs pending the determination of proved reserves |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Reclassifications to proved properties                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Ending balance                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
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|Number of wells pending determination at period-end                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       

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