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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Amount of gain (loss) recognized	$ (198)	$ (1,027)	$ (1,655)	$ 2,356
Foreign Currency Gain (Loss) [Member] | Put Options on Natural Gas Price [Member]
Amount of gain (loss) recognized		(121)		(362)
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	$ (198)	$ (887)	$ (1,655)	$ 2,832