Note 19 - Business Segments (Details Textual) $ in Thousands		12 Months Ended
		Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Number of Reportable Segments		3
Revenue from Contract with Customer, Including Assessed Tax	[1]	$ 746,044	$ 719,267	$ 692,812
Goodwill, Ending Balance		20,140	19,950	21,037
Electricity Segment [Member]
Revenue from Contract with Customer, Including Assessed Tax		540,333	509,879	465,593
Goodwill, Ending Balance		20,100	20,000	7,600
Electricity Segment [Member] | Accounting Standards Update 2014-09 [Member]
Revenue from Contract with Customer, Including Assessed Tax		61,300	26,900
Product and Energy Storage and Management Services Segments [Member]
Goodwill, Ending Balance		$ 0	$ 0	$ 13,500

[1] Revenues as reported in the geographic area in which they originate.