Note 8 - Commodity Derivative Activities - Derivative Table, Fair Value with Netting (Details) - Not Designated as Hedging Instrument [Member] - Commodity Contract [Member] - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivative Asset, Gross Amount	$ 1,084	$ 332
Derivative Asset, Amount Offset
Derivative Asset, As Presented	1,084	332
Current Assets [Member]
Derivative Asset, Gross Amount	3,035	2,893
Derivative Asset, Amount Offset	(1,209)	(2,090)
Derivative Asset, As Presented	1,826	803
Non-current Assets [Member]
Derivative Asset, Gross Amount
Derivative Asset, Amount Offset
Derivative Asset, As Presented
Current Liabilities [Member]
Derivative Asset, Gross Amount	(1,951)	(2,090)
Derivative Asset, Amount Offset	1,209	2,090
Derivative Asset, As Presented	(742)
Non-current Liabilities [Member]
Derivative Asset, Gross Amount		(471)
Derivative Asset, Amount Offset
Derivative Asset, As Presented		$ (471)