Nature of operations and summary of significant accounting policies (Accounts Receivable) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Mar. 21, 2017
Accounting Policies [Abstract]
Period within which joint interest accounts receivable are due	30 days
Period within which joint interest accounts receivable are past due	60 days
Components of accounts receivable
Joint interests	$ 16,664	$ 31,573
Accrued commodity sales	30,819	30,287
Derivative settlements	717	2,092
Other	2,544	3,375
Allowance for doubtful accounts	(1,097)	(1,240)
Accounts receivable, net	$ 49,647	$ 66,087	$ 46,837