|**Net assets from acquisition (Tables)**                                    | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|**Business Combinations [Abstract]**                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Schedule of Business Acquisitions, by Acquisition [Table Text Block]        | The following presents the preliminary values assigned to the net assets acquired as of the acquisition date:(in thousands)Total acquisition cost$326,782Recognized amounts of identifiable assets acquired and liabilities assumed:Assets acquired:Natural gas and crude oil properties - proved$126,101Natural gas and crude oil properties - unproved208,098Other assets23,589Total assets acquired357,788Liabilities assumed:Asset retirement obligations13,870Other accrued expenses10,100Other liabilities7,036Total liabilities assumed31,006Total identifiable net assets acquired$326,782
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|Total acquisition cost                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
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|Recognized amounts of identifiable assets acquired and liabilities assumed: |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Assets acquired:                                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Natural gas and crude oil properties - proved                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Natural gas and crude oil properties - unproved                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Other assets                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Total assets acquired                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Liabilities assumed:                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Asset retirement obligations                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Other accrued expenses                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Other liabilities                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Total liabilities assumed                                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Total identifiable net assets acquired                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
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                    duration X- DefinitionTabular disclosure of a material business combination completed during the period, including background, timing, and recognized assets and liabilities. This table  does not include leveraged buyouts.+ ReferencesReference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 141 -Paragraph 51 -Subparagraph a -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 141R -Paragraph F4 -Subparagraph e -Appendix FReference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=7659399&loc=d3e1392-128463Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 141R -Paragraph 68 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=7659399&loc=d3e1486-128463Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 141 -Paragraph 52 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.+ Details**Name:**
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