Note 9 - Business Segments (Details Textual) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jul. 13, 2021 USD ($)	Dec. 31, 2020 USD ($)
Number of Reportable Segments				3
Revenue from Contract with Customer, Including Assessed Tax		$ 158,842	$ 158,947	$ 472,095	$ 525,960
Goodwill, Ending Balance		91,342		91,342			$ 24,566
Two Contracted Geothermal Assets in Nevada [Member]
Goodwill, Ending Balance	[1]					$ 66,600
Electricity Segment [Member]
Revenue from Contract with Customer, Including Assessed Tax		142,651	123,660	421,503	395,201
Goodwill, Ending Balance		86,700	20,100	86,700	20,100
Energy Storage and Management Services [Member]
Goodwill, Ending Balance		4,600	3,500	4,600	3,500
Energy Storage and Management Services [Member] | Two Contracted Geothermal Assets in Nevada [Member]
Goodwill, Acquired During Period		66,600
Product Segment [Member]
Revenue from Contract with Customer, Including Assessed Tax		10,527	29,625	26,580	120,737
Goodwill, Ending Balance		0	0	0	0
Accounting Standards Update 2014-09 [Member] | Electricity Segment [Member]
Revenue from Contract with Customer, Including Assessed Tax		$ 19,000	$ 15,000	$ 59,000	$ 47,400

[1] Goodwill is primarily related to the expected synergies and potential cost savings in operations as a result of the purchase transaction. The goodwill is allocated to the Electricity segment and is deductible for tax purposes pending the exercise of the financial lease buy-out option as described below.