Nature of operations and summary of significant accounting policies and going concern (Credit Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total					$ 43,197	$ 50,744
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at January 1, 2020	$ 4,215		$ 4,215		4,215	1,097
Credit losses	1,447	$ (18)	2,964	$ (276)
Write-offs			0
Recoveries			0
Balance at June 30, 2020	4,215		4,215
Cumulative Effect, Period of Adoption, Adjustment
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at January 1, 2020						154
Commodity sales
Segment Reporting Information [Line Items]
Total					14,416	30,819
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at January 1, 2020	213		213		213	0
Credit losses			59
Write-offs			0
Recoveries			0
Balance at June 30, 2020	213		213
Commodity sales | Cumulative Effect, Period of Adoption, Adjustment
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at January 1, 2020						154
Joint interests
Segment Reporting Information [Line Items]
Accounts which have sufficient related revenue distributions payable to offset entire receivable balance					258
Accounts which have related revenue distributions payable but not sufficient to offset entire receivable balance					3,711
Accounts without related revenue distributions payable					6,023
Total					9,992	16,664
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at January 1, 2020	4,002		4,002		4,002	1,097
Credit losses			2,905
Write-offs			0
Recoveries			0
Balance at June 30, 2020	4,002		4,002
Joint interests | Cumulative Effect, Period of Adoption, Adjustment
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at January 1, 2020						0
Derivatives
Segment Reporting Information [Line Items]
Total					15,540	717
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at January 1, 2020	0		0		0	0
Credit losses			0
Write-offs			0
Recoveries			0
Balance at June 30, 2020	0		0
Derivatives | Cumulative Effect, Period of Adoption, Adjustment
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at January 1, 2020						0
Other
Segment Reporting Information [Line Items]
Total					3,249	2,544
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at January 1, 2020	0		0		$ 0	0
Credit losses			0
Write-offs			0
Recoveries			0
Balance at June 30, 2020	$ 0		$ 0
Other | Cumulative Effect, Period of Adoption, Adjustment
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at January 1, 2020						$ 0