Note 5 - 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	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Amount of gain (loss) recognized	$ (1,027)	$ 951	$ 2,356	$ (1,272)
Foreign Currency Gain (Loss) [Member] | Put Options on Natural Gas Price [Member]
Amount of gain (loss) recognized	(121)		(362)
Foreign Currency Gain (Loss) [Member] | Call Option on Natural Gas Price [Member]
Amount of gain (loss) recognized		32		(1,114)
Foreign Currency Gain (Loss) [Member] | Call and Put Options on Oil Price [Member]
Amount of gain (loss) recognized		230		(1,312)
Foreign Currency Gain (Loss) [Member] | Contingent Considerations [Member]
Amount of gain (loss) recognized	(19)		(114)
Foreign Currency Gain (Loss) [Member] | Currency Forward Contracts [Member]
Amount of gain (loss) recognized	$ (887)	$ 689	$ 2,832	$ 1,154