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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Amount of gain (loss) recognized	$ 1,364	$ (3,912)	$ 3,383	$ (2,223)
Foreign Currency Gain (Loss) [Member] | Put Options on Natural Gas Price [Member]
Amount of gain (loss) recognized	(48)		(241)
Foreign Currency Gain (Loss) [Member] | Call Option on Natural Gas Price [Member]
Amount of gain (loss) recognized		(1,664)		(1,146)
Foreign Currency Gain (Loss) [Member] | Call and Put Options on Oil Price [Member]
Amount of gain (loss) recognized		(899)		(1,542)
Foreign Currency Gain (Loss) [Member] | Contingent Considerations [Member]
Amount of gain (loss) recognized	(45)		(95)
Foreign Currency Gain (Loss) [Member] | Currency Forward Contracts [Member]
Amount of gain (loss) recognized	$ 1,457	$ (1,349)	$ 3,719	$ 465