Nature of operations and summary of significant accounting policies (Accounts receivable) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	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	$ 31,573	$ 29,032
Accrued commodity sales	30,287	26,516
Derivative settlements	2,092	157
Other	3,375	5,326
Allowance for doubtful accounts	(1,240)	(668)
Accounts receivable, net	$ 66,087	$ 60,363	$ 46,837