Nature of operations and summary of significant accounting policies (Joint Venture) (Details) - Joint Development Agreement $ in Thousands	Sep. 25, 2017 USD ($) a well
Joint Venture [Line Items]
Percentage of working interest in wells	15.00%
Percentage of working interest in wells upon achievement of required internal rate of return	75.00%
STACK
Joint Venture [Line Items]
STACK position development area | a	110,000
Canadian
Joint Venture [Line Items]
Number of joint venture stack wells	17
Garfield
Joint Venture [Line Items]
Number of joint venture stack wells	13
Bayou City Energy Management, LLC
Joint Venture [Line Items]
Funded percentage of drilling completion and equipment costs	100.00%
Number of joint venture stack wells	30
Percentage of working interest in wells	85.00%
Percentage of internal rate of return	14.00%
Percentage of working interest in wells upon achievement of required internal rate of return	25.00%
Bayou City Energy Management, LLC | Minimum
Joint Venture [Line Items]
Average well cost caps per gross well | $	$ 3,400
Bayou City Energy Management, LLC | Maximum
Joint Venture [Line Items]
Average well cost caps per gross well | $	$ 4,000