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Nature of operations and summary of significant accounting policies (Unevaluated Oil and Natural Gas Properties) (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Accounting Policies [Abstract]          
Impairment of oil and gas assets $ 147,686 $ 0 $ 261,001 $ 0  
Capitalized Costs of Unproved Properties Excluded from Amortization, Cumulative [Abstract]          
Leasehold acreage 338,892   338,892   $ 427,206
Capitalized interest 15,469   15,469   11,377
Wells and facilities in progress of completion 19,400   19,400   28,033
Total unevaluated oil and natural gas properties excluded from amortization $ 373,761   $ 373,761   $ 466,616