Note 8 - Property, Plant and Equipment and Construction-in-process (Details Textual) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Depreciation, Total	$ 98.8	$ 94.8	$ 95.2
Depreciation Net Of Amortization Of Cash Grant	5.5	5.5	$ 5.5
Property, Plant and Equipment Including Construction In Progress, Net	1,447.4	1,376.1
Property, Plant and Equipment, Cash Grant, Net	133.2	138.7
Geotermica Platanares [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 140.3	67.5
Power Plant Usage Agreement Term	15 years
Geothermie Bouillante SA (“GB”) [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 24.9	20.3
Orzunil I de Electricidad, Limitada [Member]
Property, Plant and Equipment Including Construction In Progress, Net	9.9	12.2
Ortitlan Limitada [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 40.7	$ 40.3
Kenya Power and Lighting Co Limited [Member]
Power Purchase Agreements Term	20 years
Zunil Power Plant In Guatemala [Member] | Instituto Nacional de Electrificacion (INDE) [Member]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners		3.00%
ENEE [Member] | Geotermica Platanares [Member]
Power Purchase Agreements Term	30 years
EDF [Member] | Geothermie Bouillante SA (“GB”) [Member]
Power Purchase Agreements Term	15 years
Foreign Countries [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 580.8	$ 487.0
KENYA | Power Plants [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 326.1	$ 315.0