Fair value measurements - Roll forward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Total purchases and settlements of commodity derivative deferred premiums:
Derivatives, deferred premium paid	$ 7,200
Deferred Premiums
Changes in assets classified as Level 3 measurements
Balance of Level 3 at beginning of year	(16,565)	$ (28,683)	$ (8,998)
Change in net present value of derivative deferred premiums	(139)	(694)	(394)
Total purchases and settlements of commodity derivative deferred premiums:
Purchases	0	(7,523)	(25,733)
Settlements	16,227	20,335	6,442
Balance of Level 3 at end of year	$ (477)	$ (16,565)	$ (28,683)