Note 8 - Property, Plant and Equipment and Construction-in-process - Property, Plant and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, plant and equipment	$ 3,097,806	$ 2,717,797
Asset retirement cost	11,448	10,563
Less accumulated depreciation	(1,138,228)	(983,106)
Property, plant and equipment, net	1,959,578	1,734,691
Land [Member]
Property, plant and equipment	38,060	32,178
Leasehold Improvements [Member]
Property, plant and equipment	5,718	3,984
Machinery and Equipment [Member]
Property, plant and equipment	208,646	182,121
Office Equipment [Member]
Property, plant and equipment	35,708	31,128
Automobiles [Member]
Property, plant and equipment	22,074	12,596
Geothermal And Recovered Energy Generation Power Plants [Member] \| UNITED STATES
Property, plant and equipment	2,065,377	1,744,728
Geothermal And Recovered Energy Generation Power Plants [Member] \| Foreign Countries [Member]
Property, plant and equipment	$ 710,775	$ 700,498

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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