Note 16 - Supplemental Oil and Gas Disclosures (Unaudited) - Future Net Cash Inflows (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Future cash inflows		$ 2,876,976	$ 2,035,619	$ 999,716
Future production costs		(849,063)	(609,921)	(357,917)
Future development costs		(547,163)	(461,619)	(267,836)
Future income tax expense (1)	[1]	(181,224)	(83,915)
Future net cash flows		1,299,526	880,164	373,963
Discount		(647,642)	(474,423)	(213,363)
Standardized Measure of discounted future net cash relating to proved reserves		$ 651,884	$ 405,741	$ 160,600	$ 197,251

[1] There was no provision for future income tax expense for the year ended December 31, 2016 due to net operating loss carryovers.