Note 8 - Business Segments (Details Textual) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Number of Reportable Segments			3
Revenue from Contract with Customer, Including Assessed Tax	$ 174,900	$ 184,065	$ 367,013	$ 383,103
Goodwill, Ending Balance	20,120		20,120		$ 20,140
Electricity Segment [Member]
Revenue from Contract with Customer, Including Assessed Tax	128,685	129,079	271,541	271,987
Goodwill, Ending Balance	20,100	20,200	20,100	20,200
Electricity Segment [Member] | Accounting Standards Update 2014-09 [Member]
Revenue from Contract with Customer, Including Assessed Tax	15,600	32,400	12,900	29,900
Product and Energy Storage and Management Services Segments [Member]
Goodwill, Ending Balance	$ 0	$ 0	$ 0	$ 0