Note 8 - Business Segments (Details Textual) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Number of Reportable Segments	3
Revenue from Contract with Customer, Including Assessed Tax	$ 192,113	$ 199,038
Goodwill, Ending Balance	19,963		$ 20,140
Electricity Segment [Member]
Revenue from Contract with Customer, Including Assessed Tax	142,856	142,908
Goodwill, Ending Balance	20,000	20,100
Electricity Segment [Member] | Accounting Standards Update 2014-09 [Member]
Revenue from Contract with Customer, Including Assessed Tax	16,800	17,000
Product and Energy Storage and Management Services Segments [Member]
Goodwill, Ending Balance	$ 0	$ 0