Note 9 - Business Segments - Summarized Financial Information Concerning Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2021		Jun. 30, 2020		Jun. 30, 2021		Jun. 30, 2020		Dec. 31, 2020
Revenues		$ 146,901		$ 174,900		$ 313,253		$ 367,013
Operating income (loss)		28,619		48,075		78,508		109,132
Segment assets at period end		3,818,550	[1],[2]	3,418,377	[1],[2]	3,818,550	[1],[2]	3,418,377	[1],[2]	$ 3,888,987
Segment Reconciling Items [Member]
Revenues	[3]	0		0		0		0
Segment assets at period end		103,890		84,414		103,890		84,414
Electricity Segment [Member]
Revenues		133,864		128,685		278,852		271,541
Operating income (loss)		30,018		45,875		77,767		104,505
Segment assets at period end	[1],[2]	3,530,256		3,150,770		3,530,256		3,150,770
Electricity Segment [Member] | Segment Reconciling Items [Member]
Revenues	[3]	0		0		0		0
Segment assets at period end		103,890		84,414		103,890		84,414
Product Segment [Member]
Revenues		7,410		43,701		16,053		91,112
Operating income (loss)		(427)		3,803		(1,638)		7,675
Segment assets at period end	[1],[2]	122,868		190,474		122,868		190,474
Product Segment [Member] | Segment Reconciling Items [Member]
Revenues	[3]	51,038		50,453		76,372		59,109
Segment assets at period end		0		0		0		0
Other Segments [Member]
Revenues		5,627		2,514		18,348		4,360
Operating income (loss)		(972)		(1,603)		2,379		(3,048)
Segment assets at period end	[1],[2]	165,426		77,133		165,426		77,133
Other Segments [Member] | Segment Reconciling Items [Member]
Revenues	[3]	0		0		0		0
Segment assets at period end		0		0		0		0
UNITED STATES
Revenues	[4]	93,838		85,210		207,388		179,146
UNITED STATES | Electricity Segment [Member]
Revenues	[4]	87,564		80,427		186,540		172,119
UNITED STATES | Product Segment [Member]
Revenues	[4]	647		2,269		2,500		2,667
UNITED STATES | Other Segments [Member]
Revenues	[4]	5,627		2,514		18,348		4,360
Non-US [Member]
Revenues	[5]	53,063		89,690		105,865		187,867
Non-US [Member] | Electricity Segment [Member]
Revenues	[5]	46,300		48,258		92,312		99,422
Non-US [Member] | Product Segment [Member]
Revenues	[5]	6,763		41,432		13,553		88,445
Non-US [Member] | Other Segments [Member]
Revenues	[5]	$ 0		$ 0		$ 0		$ 0

[1] Electricity segment assets include goodwill in the amount of $20.2 million and $20.1 million as of June 30, 2021 and 2020, respectively. Energy Storage segment assets include goodwill in the amount of $4.6 million and none as of June 30, 2021 and 2020, respectively. No goodwill is included in the Product segment assets as of June 30, 2021 and 2020.

[2] Including unconsolidated investments

[3] Intersegment revenue are fully eliminated in consolidation.

[4] Electricity segment revenues in the United States are all accounted under lease accounting except for $20.8 million and $40.0 million for the three and six months ended June 30, 2021, respectively, and $15.6 and $32.4 million for the three and six months ended June 30, 2020 , respectively, that are accounted under ASC 606. Product and Energy Storage segment revenues in the United States are accounted under ASC 606.

[5] Electricity segment revenues in foreign countries are all accounted under lease accounting. Product segment revenues in foreign countries are accounted under ASC 606.