Note 16 - Supplemental Oil and Gas Disclosures (Unaudited) - Changes in the Standardized Measure (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Standardized Measure, beginning of year	$ 405,741	$ 160,600	$ 197,251
Sales and transfers of oil and gas produced, net of production costs	(112,707)	(63,764)	(32,834)
Net change in prices and development and production costs from prior year	268,942	159,661	(58,425)
Extensions, discoveries, and improved recovery, less related costs	153,544	129,277	5,531
Sales of minerals in place	(39,253)	(8,583)	(4,433)
Purchases of minerals in place	8,990	1,238
Revisions of previous estimates	(67,345)	31,044	12,317
Change in timing and other	30,811	1,908	21,468
Change in future income tax expense	(37,413)	(21,700)
Accretion of discount	40,574	16,060	19,725
Standardized Measure, end of year	$ 651,884	$ 405,741	$ 160,600