Nature of operations and summary of significant accounting policies (Cash and Accounts Receivable) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Cash held	$ 29,695
Components of accounts receivable
Joint interests	25,713	$ 31,573
Accrued commodity sales	25,318	30,287
Derivative settlements	1,120	2,092
Other	1,538	3,375
Allowance for doubtful accounts	(1,003)	(1,240)
Accounts receivable, net	$ 52,686	$ 66,087