Note 8 - Property, Plant and Equipment and Construction-in-process - Property, Plant and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, plant and equipment	$ 2,717,797	$ 2,436,858
Asset retirement cost	10,563	8,669
Less accumulated depreciation	(983,106)	(880,480)
Property, plant and equipment, net	1,734,691	1,556,378
Land [Member]
Property, plant and equipment	32,178	31,904
Leasehold Improvements [Member]
Property, plant and equipment	3,984	3,848
Machinery and Equipment [Member]
Property, plant and equipment	182,121	152,821
Office Equipment [Member]
Property, plant and equipment	31,128	28,634
Automobiles [Member]
Property, plant and equipment	12,596	11,161
Geothermal And Recovered Energy Generation Power Plants [Member] \| UNITED STATES
Property, plant and equipment	1,744,728	1,658,195
Geothermal And Recovered Energy Generation Power Plants [Member] \| Foreign Countries [Member]
Property, plant and equipment	$ 700,498	$ 541,626

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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