Note 8 - Property, Plant and Equipment and Construction-in-process (Details Textual) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 22, 2014
Depreciation, Total	$ 126.7	$ 114.4	$ 98.8
Depreciation Net Of Amortization Of Cash Grant	7.3	6.4	$ 5.5
Property, Plant and Equipment Including Construction In Progress, Net	1,841.4	1,696.4
Property, Plant and Equipment, Cash Grant, Net	162.3	179.7
Geotermica Platanares [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 96.1	105.7
Power Plant Usage Agreement Term	15 years
Geothermie Bouillante SA (“GB”) [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 24.5	23.9
Orzunil I de Electricidad, Limitada [Member]
Property, Plant and Equipment Including Construction In Progress, Net	10.3	14.6
Ortitlan Limitada [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 42.8	43.5
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	$ 506.6	524.8
KENYA | Power Plants [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 284.5	$ 302.0