Note 7 - Fair Value of Financial Instruments - Amounts of Gain (Loss) Recognized in Condensed Consolidated Statements on Derivative Instruments Not Designated as Hedges (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Amount of gain (loss) recognized	$ 7,411	$ 5,268	$ (3,942)
Foreign Currency Gain (Loss) [Member] | Put Options on Natural Gas Price [Member]
Amount of gain (loss) recognized		(350)
Foreign Currency Gain (Loss) [Member] | Call Option on Natural Gas Price [Member]
Amount of gain (loss) recognized			(1,340)
Foreign Currency Gain (Loss) [Member] | Call and Put Options on Oil Price [Member]
Amount of gain (loss) recognized			(1,313)
Foreign Currency Gain (Loss) [Member] | Contingent Considerations [Member]
Amount of gain (loss) recognized	170	(129)	(1,527)
Foreign Currency Gain (Loss) [Member] | Currency Forward Contracts [Member]
Amount of gain (loss) recognized	(3,081)	3,699	238
General and Administrative Expense [Member] | Currency Forward Contracts [Member]
Amount of gain (loss) recognized	$ 10,322	$ 2,048