|**Business Combination Business Combinations (Tables)**                     | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
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|**Business Combinations [Abstract]**                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Business Combination, Segment Allocation [Table Text Block]                 | The details of the estimated purchase price and the preliminary allocation of the purchase price for the transaction, are presented below (in thousands):March 31, 2018Acquisition costs:Cash$171,091Deposit made in prior period21,000Total cash consideration192,091Other purchase price adjustments9,734Total acquisition costs$201,825Recognized amounts of identifiable assets acquired and liabilities assumed:Assets acquired:Current assets$517Crude oil and natural gas properties - proved208,279Other assets2,796Total assets acquired211,592Liabilities assumed:Current liabilities(5,080)Asset retirement obligations(4,687)Total liabilities assumed(9,767)Total identifiable net assets acquired$201,825
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|                                                                            | March 31, 2018                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Acquisition costs:                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Cash                                                                        | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Deposit made in prior period                                                | 21,000                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Total cash consideration                                                    | 192,091                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Other purchase price adjustments                                            | 9,734                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Total acquisition costs                                                     | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
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|Recognized amounts of identifiable assets acquired and liabilities assumed: |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Assets acquired:                                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Current assets                                                              | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Crude oil and natural gas properties - proved                               | 208,279                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Other assets                                                                | 2,796                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Total assets acquired                                                       | 211,592                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Liabilities assumed:                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Current liabilities                                                         | (5,080                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Asset retirement obligations                                                | (4,687                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Total liabilities assumed                                                   | (9,767                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Total identifiable net assets acquired                                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      

X- DefinitionTabular disclosure of goodwill in a business combination.
+ ReferencesReference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479
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