Basis of presentation and significant accounting policies - Fees received for the operation of jointly-owned oil and natural gas properties (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
General and administrative expense
Fees received for the operation of jointly-owned oil and natural gas properties	$ 464	$ 468	$ 412