Note 4 - Acquisition and Divestitures of Properties (Details Textual)	12 Months Ended
	Dec. 31, 2018 USD ($) a	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Proceeds from Sale of Property, Plant, and Equipment, Total	$ 26,000	$ 204,000	$ 4,022,000
Leasehold and Working Interests in Permian Basin Region [Member]
Mineral Area Acquired | a	2,721
Property, Plant and Equipment, Additions	$ 40,400,000
Capital Expenditures Incurred but Not yet Paid	21,000
Non-core Properties [Member]
Proceeds from Sale of Property, Plant, and Equipment, Total	$ 3,300,000