Note 13 - Oil and Natural Gas Producing Activities (Unaudited) (Details Textual) Mcfe in Thousands, $ in Millions		12 Months Ended
		Dec. 31, 2019 USD ($) Mcfe $ / MMBTU $ / bbl	Dec. 31, 2018 USD ($) Mcfe $ / MMBTU $ / bbl	Dec. 31, 2017 $ / MMBTU $ / bbl
Costs Incurred, Asset Retirement Obligation Incurred		$ 0.1	$ 0.1
Proved Developed and Undeveloped Reserves, Sale of Mineral in Place (Energy) | Mcfe		54,571	2,500
Oil and Gas Reclamation Liability, Noncurrent		$ 7.4	$ 7.3
Crude Oil and Natural Gas Liquids (NGL) [Member]
Oil and Gas, Average Sale Price | $ / bbl	[1]	55.69	65.56	51.34
Natural Gas and Natural Gas Liquids [Member]
Oil and Gas, Average Sale Price | $ / MMBTU	[1]	2.58	3.1	2.98

[1] These index prices, used to estimate our reserves at these dates, are before deducting or adding applicable transportation and quality differentials on a well-by-well basis.