Nature of operations and summary of significant accounting policies (Cash and Cash Equivalents and Accounts Receivable) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Cash held	$ 20,379
Components of accounts receivable
Joint interests	18,605	$ 31,573
Accrued commodity sales	23,408	30,287
Derivative settlements	2,820	2,092
Other	1,412	3,375
Allowance for doubtful accounts	(1,100)	(1,240)
Accounts receivable, net	$ 45,145	$ 66,087