Nature of operations and summary of significant accounting policies (Joint Development Agreement) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 25, 2017 USD ($) 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%
Joint Development Agreement
Joint Venture [Line Items]
Joint venture cost overages recorded	$ 4,061	$ 13,212
Bayou City Energy Management, LLC | Joint Development Agreement
Joint Venture [Line Items]
Funded percentage of drilling completion and equipment costs			100.00%
Number of joint venture stack wells | well			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 | Joint Development Agreement | Low
Joint Venture [Line Items]
Average well cost caps per gross well			$ 3,400
Bayou City Energy Management, LLC | Joint Development Agreement | High
Joint Venture [Line Items]
Average well cost caps per gross well			$ 4,000