Note 1 - General and Basis of Presentation - Summary of Pro Forma Information Related to a Business Combination (Details) - Two Contracted Geothermal Assets in Nevada [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Business Acquisition, Pro Forma Revenue	$ 160.8	$ 175.8	$ 499.7	$ 569.1
Net income	18.5	18.0	50.4	64.3
Electricity Segment [Member]
Business Acquisition, Pro Forma Revenue	$ 144.7	$ 140.5	$ 449.1	$ 438.4