Note 9 - Business Segments (Details Textual) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Number of Reportable Segments			3
Revenue from Contract with Customer, Including Assessed Tax	$ 146,901	$ 174,900	$ 313,253	$ 367,013
Goodwill, Ending Balance	24,863		24,863		$ 24,566
Electricity Segment [Member]
Revenue from Contract with Customer, Including Assessed Tax	133,864	128,685	278,852	271,541
Goodwill, Ending Balance	20,200	20,100	20,200	20,100
Electricity Segment [Member] | Accounting Standards Update 2014-09 [Member]
Revenue from Contract with Customer, Including Assessed Tax	20,800	15,600	40,000	32,400
Energy Storage and Management Services [Member]
Goodwill, Ending Balance	4,600	0	4,600	0
Product Segment [Member]
Revenue from Contract with Customer, Including Assessed Tax	7,410	43,701	16,053	91,112
Goodwill, Ending Balance	$ 0	$ 0	$ 0	$ 0