Note 11 - Income Taxes - Components of Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Loss carryforwards U.S. - Federal/States	$ 17,073	$ 13,500
Asset retirement obligations	3,306	3,178
Accrued expenses	521
Equity-based compensation	1,393	803
Other	4
Total deferred tax assets	22,297	17,481
Less valuation allowance		(4,464)
Deferred tax assets, net	22,297	13,017
Deferred tax liabilities:
Depreciation & amortization	(22,406)	(13,017)
Net deferred tax liabilities, net of valuation allowance	$ (109)