Note 9 - Property, Plant and Equipment and Construction-in-process (Details Textual) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Depreciation	$ 94.8	$ 95.2	$ 87.9
Depreciation Net Of Amortization Of Cash Grant	5.5	5.5	$ 5.3
Property, Plant and Equipment Including Construction In Progress, Net	1,376.1	1,335.0
Property, Plant and Equipment, Cash Grant, Net	138.7	144.2
Orzunil I de Electricidad, Limitada [Member]
Property, Plant and Equipment Including Construction In Progress, Net	12.2	19.2
Ortitlan Limitada [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 40.3	46.0
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%
Foreign Countries [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 487.0	473.1
KENYA | Power Plants [Member]
Property, Plant and Equipment Including Construction In Progress, Net	$ 315.0	$ 355.8