Note 8 - Property, Plant and Equipment and Construction-in-process (Details Textual) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 22, 2014
Depreciation, Total	$ 114.4	$ 98.8	$ 94.8
Depreciation Net Of Amortization Of Cash Grant	6.4	5.5	$ 5.5
Property, Plant and Equipment Including Construction In Progress, Net	1,696.4	1,447.4
Property, Plant and Equipment, Cash Grant, Net	179.7	133.2
Geotermica Platanares [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 105.7	140.3
Power Plant Usage Agreement Term	15 years
Geothermie Bouillante SA (“GB”) [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 23.9	24.9
Orzunil I de Electricidad, Limitada [Member]
Property, Plant and Equipment Including Construction In Progress, Net	14.6	9.9
Ortitlan Limitada [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 43.5	40.7
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] | Orzunil I de Electricidad, Limitada [Member]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners				3.00%
ENEE [Member] | Geotermica Platanares [Member]
Power Plant Usage Agreement 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	$ 524.8	580.8
KENYA | Power Plants [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 302.0	$ 326.1